To get minutes, a player basically needs to beat out the one ahead of him/her.
It's frustrating watching an inconsistent rookie play while a journeyman pro sits, but there is little reason for a rookie to sit while a minimum contract fringe NBA player who is a touch more "complete" or experienced, and consequently perhaps a bit "better."
I mean, Rondo had to beat out Telfair and West to get PG minutes. Thankfully, he did.