I don't know if you can treat Purdy's experience equally when it was under a totally different staff asking him to do different things. Personally, I'd like to see him get a shot for at least a series or two - I think Rhule said something along the lines of "he throws the ball the best" in the fall. Our offense has just been so deficient in the passing game that I think even if Purdy is worse than Haarberg overall, if he is a step up in the passing game I would buy that the offense could be better. I'd trade some of our run game for more of a pass game.
That being said, I'm sure the coaches have considered that and don't feel Purdy's edge in the passing game outweighs Haarberg's in the run game. I'd still like to see, but rotating QBs is tough.