Have ANY games done it besides FPS?

Which has the architecture of an FPS.

In other words, P2P games can't allow this

No, they can't allow it, not without sending the equivalent of an entire save game. Which means a big pause for everyone currently playing the game.

i think it would be too unbalanced for thing like the Pantheon.  say ur in a game and ur obviouslt more skilled then they are... overpowering them till late game.  then everyone rage quits and brand spaking new bunch of hardcore pro players join resulting in u being viciously raped by their super pro skillz.  thus all the work u did in the early game is for not because when the new people join its basicly another game.

i do think however that a Switch Side command might be interested if your going to take DotA as an example.  sure it annoying as hell when ur most fed player jumps ship and solo's ur whole team, but at least it saves u from the dam Normal AI replacements.