Re: Does your GM play too?
I run a Warhammer Fantasy campaign right now that is very plot driven, so I use a GMPC as a stop gap, but I designed him very carefully. He's a mute, illiterate halfling who can only communicate through crude sign language. He stubbornly refuses to learn to read/write. This keeps him from being the "go to guy" when players are stuck and, being a halfling, he's not much use in a fight. He's also one of the PC's valet, so he's only there for comic relief (my players get great fun out of watching me wave my hands around) and occasionally my Deus Ex Machina. For instance, my party was trying to climb down a prison wall to escape and were having a hard time until my halfling tapped one of the Dwarves on the shoulder and pointed to the rope & grapple on his belt. (Yeah I have one of those players who doesn't know his yass from his elbow from his character sheet.)
Anyway, GMPCs can be helpful, if used sparingly and carefully, but I can see why some GMs avoid them.
God knows I've been apart of more then one campaign where the GM was only using the game as a venue to show off his NPCs and let them have center stage, which was, of course, lame.