Ever seen them? I have seen a number of different types of DMPCs ranging from the DM admitting he is playing his character while he DMs to a super NPC the DM is running just because he thinks the NPC will be an asset to the party.
The first variety of DMPC is probably the rarest nowadays, but is more honest. We used to see them more when we had a semi-rotating DM seat and the current DM didn't want his character losing out on XP or for the party to lose his abilities for the duration of the adventure. My friend Tim McD. used his Dwarven Thief extensively during the campaign he ran when I was in high school. There are still problems with this style of DMPC though. He will consciously or unconsciously favor his PC, while at the same time giving him the benefits of NPC-hood, like not being targeted during most combats because he hangs back like a good specialist NPC.
The second type of DMPC is where they are dishonestly promoting an NPC (or designing one specifically for the purpose) to essentially be their own PC. Usually they do this to fill in a specified niche or a perceived hole in the party. Sometimes they do it for the express purpose of making sure certain story lines get followed. In my experience players universally hate them.
Now, I have had NPCs that filled specific roles, often a party cleric's role has fallen to me as DM. Occasionally party mage, though more often they just make due without. I try not to, but sometimes the party needs a hint because they failed to pick up a bit of information I dropped so I will re-hint using a wise party NPC. I had one campaign where the party had a powerful wizard as a patron and he saved their bacon on more than one occasion. I have also run my own PC as an NPC back during the rotating DM slot days. So, as a DM, I am not completely immune to the very things I am complaining about here.
My problem is the abuses. I have three stories that show the worst examples I have ever witnessed. The first is a story of Tim's DMPC (and occasional PC) the Dwarven Thief Andemon. Andemon, in his NPC guise, was the head of the thieves guild in Specularum, the port town we used as a base of operations in his campaign. Andemon also was the owner of Andemon's Armory, the only place in town where you could purchase magic items, or sell them for that matter. Andemon also was a regular adventuring companion and kind of served as a kind of foreman for the large number of NPC thieves that we regularly took on as our hirelings. The NPC thieves were more loyal to their guildmaster than to their employers. This turned to my distinct disadvantage once.
I brought in a character I usually didn't play in this campaign, but my PC Lodor (an Elven F/M-U) was in need to replenish his fortunes after a failed expedition that nearly bankrupted him in another campaign I played in. Jumping from one DM's campaign to another DM's was fairly common in those days, you just usually had the DM check out your sheet and make sure your PC was compatible with his campaign, by which I mean of course that you were neither an egregious cheater nor coming in from a Monty Haul situation. It was the DM's judgment in those days, no treasure quotas or point buy builds.
Anyway, Lodor had been pretty well screwed over in another campaign and needed to make some money and get some swag, so I asked Tim if it would be OK if I played him the next time he DMed. He said sure and I thought things would be all right. I could not have been more wrong. Lodor had to hock his on valuable thing he had left to finance himself for this expedition. No one had been willing to extend credit to him, despite the fact that I had played with these guys for a couple of years at this point, they didn't know my character. Fair enough. However, I was also subjected to the most racism I have ever seen in a D&D game. I have mentioned before that Tim's campaign was Dwarf heavy and Elf shy based on our limited selection of miniatures. These Dwarves made no bones about the fact that they hated Elves. I have to say I hadn't seen it coming. No one had ever tried to play an Elf in Tim's game previously.
So, to finance getting new armor and weapons and a few basic adventuring supplies, I had to pawn my 97,000 GP value necklace, to the guild master thief so I could buy some gear from his shop at inflated prices. In addition to paying him back the paltry 1500 GP he gave me for the necklace, with interest, I had to give him one pick of my treasure items as payment. As surety for the "loan" he and a few of his guild thieves were going to go adventuring with us, which was not unusual, just bad under the circumstances.
Over the course of the adventure the Dwarves and Thieves in the party made sure I knew they didn't like Elves. One of the other players implied to me that he knew my character wasn't going to make it out of the dungeon alive. I got more and more uncomfortable playing in this game. Then Andemon took me, alone, to go scout away from the rest of the party. We encountered a neo-otyugh while we were separated from the rest. I got the feeling like this was where Andemon was going to make his move, so I made a move first. When he advanced to attack, moving before me due to his higher initiative roll, I retreated and wizard locked the door behind me. I figured if he survived the encounter I would be strong enough to take him out and then blame his death on the dangers of the dungeon and never bring my Elf PC back to this DM's game.
What happened was, he used a ring of dimension door and popped out in the hallway behind me. He then shot me with his repeating crossbow in the back 4 times using bolts poisoned with the most toxic type of contact poison. He was pissed that I had tried to take out his DMPC. He also never admitted that he had been planning to take out my "stupid elf character", although one of the other players admitted that had been the case.
Lodor, eventually, was revived to life by my buddy Darryl C's character Borg only to perish, along with Borg, in the Mines of Bloodstone (module H2). He really should never have been there, but he was still working off his debt to Borg and Elisha for resurrection, re-equipping and new spell books.
But, before you start thinking Darryl is too awesome, the next two examples come from games he ran. They don't get as much detail, because the games were one-shots. The first was a Boot Hill game that he ran for his brother Keith and I. Our Boot Hill games nearly always devolve into us "riding the outlaw trail" so to speak. Keith especially liked brawling in Boot Hill, so their were plenty of saloon brawls and we both like gunfighting and bank robbing in most of our games, but Darryl had a different plan this time. We would be Lawmen. He had us get word that the combined Wild Bunch and James gangs were headed to our town to bob the bank. We deputized as many men as we could and layed an ambush.
That's where things went wrong for us. Darryl made a "special" NPC to help us out against these super-awesome bad guys that were coming our way. I don't remember the guy's name, but Keith and I both hated him. He was a tall skinny guy with a buffalo rifle. He was faster with that long gun than I was with my quick-draw revolver and way more accurate. When the bad guys showed up he could almost have fought them alone. He was a wild west super hero. He never missed and he hit harder, one shot- one kill, every time. He only needed us there so the bad guys would have someone else to shoot at.
When the combat was over the mayor and his men came out to congratulate Mr. Awesome. That was the straw that broke the camel's back for both me and Keith. We opened fire simultaneously, shooting Awesome in the back repeatedly until we were sure he was dead. I think I reloaded and rolled his corpes over so I could empty my revolver into his face too, just to make double sure. Darryl was both pissed and flabbergasted. Then we had to flee town, killing several townsfolk in our attempt to escape and getting grievously wounded in the process. He never understood why we killed that guy.
His other example comes from a Twilight 2000 (1st edition) game he decided to run for us one night. There were like five or six of us and we killed a couple of hours making characters while Darryl read the rules and the introductory adventure for inspiration. None of us was familiar with the rules, the game had just come out. He and I had previously made characters and his dad was going to GM a campaign for us (although we never got much past the first session of that one either).
Anyway, Darryl gives us a situation report and we start heading for Krakow, like we are supposed to. He eventually decides we need to do something besides creep across the landscape from camp to camp and take days gathering supplies to make more ethanol, so he essentially give us a random encounter with a Soviet mechanized company. Our scouts spotted them and they were headed right for us, so we camouflaged our camp as best we could, then set ourselves an ambush in case they spotted us. Our ambushers got spotted piecemeal though and either bugged out or got captured. At this point, in retrospect, I am pretty sure he was just faking it because he didn't understand the combat rules and had kind of backed himself into a corner.
The room full of teenage boys he was GMing for wanted some combat action and he wasn't sure how to deliver. We were all combat monster PCs armed to the teeth, he was trying to bluff us into fleeing so he could convey the idea that we needed to fear contact with the enemy and how dangerous the world was. We could not have cared less. We didn't care about how dangerous the setting was we were bored with searching for junk. All of us were aching to lay down some serious ass-kicking. We had grenade launchers and ATGMs and sniper rifles and machine guns. We craved action. So of course he captured us all using overwhelming force and we got to spend an hour or so of game time as Soviet prisoners. Nothing we did to escape or negotiate our release even started to work before ultimately failing.
At that point I think he realized he had lost us all. So he did the worst thing he could have done under the circumstances. He brought in a super team of allied commandos to rescue us, which was bad enough, but then they took command of our unit because they outranked us too. Shortly after our rescue we realized that they were immune to everything. If a combat broke out they didn't roll any dice, Darryl just described the effects of their actions. My character, a well trained martial artist, got his ass kicked by their team pansy in a fight over some spoils on the battlefield. I maintain that my PC needed the ammo more because I actually expended ammo on to-hit rolls. We all quit playing after that; calling an early night that night. Then we just played Dawn Patrol for the rest of the weekend. Yes, Darryl made a group of teenage boys from the 80's think that Twilight 2000 sucked.
I guess what I am trying to say here, in my typical long-winded conversational style, is that NPCs need to know their place in a party and DMs need to keep them there. Otherwise we get Elminster of Shadowdale or Drizzt Do'Urden showing up and grabbing all the glory. Everyone over a certain age hates the Forgotten Realms and 2nd edition AD&D for just this reason. I don't think that most people play RPGs so they can be second fiddle to some super NPC. Beware the impulse to play a character in your own games. Tim comes off looking pretty bad here and he wasn't usually, the story about him is an exception to how he usually was as a DM. Darryl comes off looking pretty bad too I guess, but both of the stories about him are just the worst examples of how he typically DMed. He was always long on good adventure ideas, but seemed to want to play the starring role himself too.