Some teams like to stockpile QB's

to stop everyone else having a shot. The Raiders have 2 possibly 3 that could start elsewhere, the Bears have 2 - though some would say that Orton is a better bet than the others.
I guess football has always been that way - and its not just QB's that get hurt and a backup has to takeover.
When the Texans lost Ahamn Green they had to turn to Ron Dayne.....
The speed of the NFL is something different from the college game and some cant live with the adjustment. Most of the QB's physically have the talent to manage most NFL offences' but what separates the good from the ordinary is swiftness in making decisions and adjustments. These are the ones that never seem to get hurt, who get rid of the ball just that split second quicker.
Brady and Manning are the best examples of that.