Right. Deep benches win you more regular season games- and can be the difference between a win and a loss, but once the rotations tighten, those players may not even see the floor.
Very few teams have enough talent that one of their best players starts on the bench. Typically your best starting five includes all your best players. If a team has a bench player that would be a starter elsewhere, he's a young player, or someone the team will soon loose to free agency. Even if you're good enough to draft well, you can only hold on to them if they're willing to deal with your team and sacrifice some money for the chance to win titles.
Rest becomes important even for the best well-conditioned athlete. A solid bench can buy the starters a few extra minutes that will help them to be rested enough to compete strongly at the end of the game.