Good idea, or maybe something could be done in the opposite direction, as in, have a QB get slightly better as he "get's into the flow of a game". If he get's taken out, he goes back to being "cold".
If that's done though, maybe there could be a rule for when to bench the starter when he's having a bad game, such as TD/INT ratio, or QB rating at certain points in the game. Maybe even tie it into the Head Coach or OC's ratings with a rating for "pull QB" or something.
Just thinking out loud, (so to speak/type).