For a position that is so important to a good football team, it is amazing how bad teams are at drafting quarterbacks.
Interesting analysis has been done on QBs and their transition into the pros and, so far, it seems indicative that keeping a quarterback out for his first season or two generally leads to more success than just throwing him into the fire. Rodgers, Rivers, and Brady are all examples of guys who sat out then stepped in and were immediately effective. Of course, there are always the exceptions such as Payton Manning and Ben Roethlisberger.
It seems likely to me that QBs who get the opportunity to sit benefit in two ways. First, they get to see how a veteran works, work with the guy in practice, and generally gain NFL knowledge before they step on the field. Second, I have a feeling that if we looked at teams where their now-starting QB sat his first one or two seasons, we'd find that they were generally decent football teams. They aren't in shambles like Buffalo is today or St Louis was a few years ago, so when they actually get an opportunity to start, they're stepping into a team that is primed for mediocrity or better, not epic failure.