It's true that back-ups are extrememly important for the times in the season where you are playing 3 meaningful games a week. But once you get to two games + scrimmage, their importance goes down significantly. For example the top team in my league has trash back-ups for C and PF but when I watch games against him my team never seems to be able to go on runs with the back-ups in. It also depends on stamina, if your starters have average or above stamina it seems to me that they play more minutes during the game. I have a SF with proficient stamina and a couple of times he has played 46-48 minutes even when I have a back-up assigned in the line-up. Still, if he is playing 46-48 minutes twice a week, his gameshape will suffer