The Rise of the GMPC?

(Day 2! Fingers crossed.)

I have an account on the RPG Stack Exchange. It's a great resource if you have rules questions for an RPG. It's also pretty entertaining to browse questions for games that you don't play. I've found some pretty interesting games that way, like Fiasco and Dogs in the Vineyard. Lately, though, I've been seeing some worrisome questions show up there. They involve the GMPC.

The GMPC is a player character that's controlled by the Game Master. It's separate from an NPC (non-player character), in that the GMPC presumably creates this character the same way that the players do, and then proceeds to be a part of the party, fighting monsters, solving puzzles, and gaining treasure/XP like the players do.

I am absolutely baffled by how this would come about. As someone who has been a Game Master for a vast majority of their gaming career, the very idea of this is anathema to me. A Game Master has a literally unlimited number of NPCs that they control. They can create those NPCs to be any power level, and give them whatever they would like in terms of abilities and treasure. With all this at their fingertips, why would a GM need a GMPC?

I can sort of see the evolution of the idea. I've had NPCs that have turned in Mary Sues because I was too attached to them. Or I decided that my NPC couldn't be defeated by the players by any means, and could do whatever they wanted it. It's an ego trap that a lot of GMs fall into and I understand it well.

However, to take it one step further and actually make a character to travel with the rest of the party...What exactly is the point of that? As the DM, you know what the encounters are going to be, you know any secrets that the plot has, and you know where the bodies are buried. You can't exactly role-play with NPCs because you'd be playing both characters. You can't solve puzzles for the party, because you already know the answers.  What would you actually do as a GMPC, other than control your character in combat?

I'm going to go out on a limb here, and say that I believe the rise of the GMPC is because of the effect that video games have had on RPGs. I think that new players see the role of GM as boring and they want to take part in the same excitement that the other players are having. So they think "I'll make a character as well!"

Looking at the questions on RPG.SE, it seems like GMs are trying to help a party that they deem as 'too weak'. Whether it's having a small number of players, or lacking a specific party role (like leader, healer or tank), they want to try to fill that gap with an additional player. I see at least two problems here. One, a GM should have a good enough grasp of the game to approximate how difficult an encounter is. Yes, this is a skill that's learned through a lot of GM experience, but even a brand new GM should be able to prevent a total party kill (TPK) if they don't want it to happen.

Two, there's nothing that a GMPC can do that an NPC can't do as well. If the party needs a healer and nobody wants to play one, then they should try to recruit one inside the game itself or try their luck without one. An NPC healer serves the exact game purpose as a GMPC healer would. If the PCs wouldn't trust an NPC because of the GMs track record in running other NPCs, there's a bigger issue with the GMs style.

I don't want to get into too much "one-true-wayism", but I feel that a GMPC is ALWAYS a bad idea. An NPC can handle whatever role a GMPC would serve, and, by definition, an GM should be less attached to an NPC than they would be a GMPC. The desire to have an "awesome" GMPC can lead the GM down a slippery slope that can quickly kill a game if left unchecked. It's really just bad GMing taken to a  new level.