I think this is actually not a bad idea; however, instead of taking back a move in a game you lost, a better idea would be to allow easy import of a FEN into the "Set Piece" interface. Then, make the "analyze board" feature print out the FEN as you step through the game.
That way, if you or your partner makes a screwy move, or if you just want to replay from a certain position, you can open up the game, move backwards until you have the position you want, copy the FEN, and then import this FEN into a new game.
So, it's not really taking back a move, but just letting you replay a position and avoid a mistake. Since the new game is a set piece, it won't affect ratings.
What do you think?