Frazier is beyond the age where he can be considered a prospect. You can argue that he has earned the starting third base job and maybe he has, but at his age he needs to be in the bigs even if he sits.
If he can prove he can come off the bench cold and play well, he will have a spot on the 25 man roster secured for next year even if we acquire another starting third baseman.
HRod, though, needs to play every day.