Iron Captain said:
On the one hand I think good NPCs are what makes roleplaying really fun. On the other hand when NPCs get into combat they tend to drift into the background until one player finally asks "Hey what happened to NPC?"
DM:"Oh uhmmm he's right here."
In my experience DMs tend to forget about NPCs in combat rather than push them into the spotlight.
Something similar happens to me - I remember their existence, I just make sure they stay
out of combat. I never use combative GMPCs.
I've used something that could resemble a GMPC twice.
The first time, the heroes were short on people (a few people couldn't make it) and didn't have a medic. I let them make an Ordinary (think expert) with two fewer levels than them to accompany them. They tried to keep him out of combat, NPCs didn't deliberately target him (they didn't perceive him as a threat), but twice he nearly got killed by traps! (I let the heroes control his actions that time.)
The second time, the PCs were on a jungle trek, and they had a few NPCs going with them. One, Dr. K, turned out to be almost "too good" - statistically he wasn't better than the PCs in any way, but he knew more about the natives than any of the PCs and could speak their language, which meant he ended up doing quite a bit of social interaction that should have been left to the PCs. Whoops. That's something I'll try not to do again. (One of the natives could speak English ... poorly ... but speaking to him took about three times longer than speaking to someone who understood English well.)
I've scrapped every other GMPC concept I came up with, because they always end up being better than the PCs once I've designed them - I'm a good optimizer when I put my mind to it.