The New York Giants of the mid-1980s were a classic example of this . . . Jeff Rutledge was the backup to Phil Simms, while Jeff Hostetler was drafted in the hopes that he would be the future of the franchise -- and spent most of his first few years as the #3 quarterback.
The salary cap doesn't allow teams to have that kind of depth anymore . . . so the rookie becomes the #2 quarterback, and the #3 quarterback is some guy like Vince Evans or Jeff George who has been in the league for about 70 years and sits by his phone waiting for a call from a desperate team.
Well part of that punchline is that the #3 QB spot has been largely wasted through history, it's only there for when 2 QBs get hurt. It's hard to even learn the ropes from there since it's usually the #2 guy QBing the practice squad.
The salary cap only hurts a team's depth chart if the GM is stupid. All GMs that use the cap as an excuse for a pathetic depth chart (and there are plenty) should be drummed out of the league. These are GMs that would have lousy depth charts any way (and there were plenty of teams with bad depth charts before the cap) and have found a convenient excuse for their own short comings.