See this is the line of thinking that always baffles me. While many of you bring up Matt Cassell and AJ Feeley types as proof that backups don't work, you swoon over a young draft choice who has never seen an NFL defense first hand. It's all about risk/reward. Andrew Luck should be a good one, but he's never played at the NFL level. RGIII should be a solid NFL QB, but if he depends on his athletic ability then he will get crushed with the level of athleticism in the NFL at the LB and DE position in today's game.
I am more inclined to take a 26 yr old QB who has shown that he can thrive in a certain scheme in the NFL without making tragic errors. Hell, Rex has shown he can do great things in the offense, his problem is he doesn't know how to adjust when plays break down and Matt Flynn has shown against a couple of good teams he can do that in the NFL. He has some mobility, more arm strength than credited, critical thinking when things break down, and intuition to scan a defense from the line of scrimmage and adjust the play accordingly.
Do I believe Matt Flynn is Tom Brady? No! He is Matt Flynn. Is he a product of the scheme? Well yes! But he is taking full advantage of his opportunities. Could he succeed in Washington? Well if Matt Schaub can succeed under Kyle (#1 offense in the NFL) then there is no reason Flynn shouldn't be at least better leadership than Rex Grossman in Washington.
Goal, you mentioned time, well I suggest to you that Shanahan does not have time to go with a rookie at QB. If we don't, that means Rex gets the nod. Why on earth would we not sign a QB who we know can come in and start, or at least has greater chance of reward than a rookie who has never seen an NFL defense or Rex who has?