For example, the PG starter to play at least 35 minutes but not more than 40 minutes.
Having this option would make sure that training would be so easy its not even fun. The same thing would go for GS.
In case we continue with your POV, you can say - the coach should discard the line-up regarding minutes sharing.
It could be anything that he would like. He can decide that the GB starts at the second half or minute to the end.
"Why make the time spreading so easy like it is now?"
There is a line that separate reasnable coach desicions, and desicions that will make the game just to easy.
When a coach does not opperate upon reason (or more precisely, the opposite from it), it means that it is needed to be fixed.