Are we talking about guys who sat the bench for a couple seasons or guys thrown to the wolves? The history of the NFL is littered with guys who got thrown out there too early and saw their careers drowned in a sea of sacks and fumbles. The league almost destroyed Troy Aikman, who, let's be honest, wouldn't have made it in today's impatient NFL. Even in that time, Jimmy brought in Steve Walsh to try and take his job. Drew Brees had a QB rating of around 70 the first three years. Elway had more INTs than touchdowns in three of his first six seasons.
As for guys who sit to start their careers? Romo, Manning, Cousins (asterisk at this point), Rogers, Brady, Brees, Warner (asterisk due to career track), and Rivers all sat. This used to be more common, but has been phased out due to the lack of patience by front offices.
As mentioned above, you are seeing less patience in developing quarterbacks in the modern NFL. The kids that come and succeed early usually have very conservative offensive play calling and a stellar ground game, but teams give up on kids far quicker today.