Or on the flip side, him being the most mobile of the 3 QBs, may give us the best shot with this (possibly) crappy OL. Additionally, the fact that he can keep defense honest by throwing deep may give enough breathing room for Shady.
When Manuel was the starting QB, the DB's dared him to throw by crowding the LoS and stopping the run. Manuel failed. Will Allen deliver on that promise. While our philosophy (overall) may be the run setting up the pass (and protecting the confidence of the rookie), Allen would have to threaten with the deep pass so as to allow the run game to flourish.