Minute management is what differentiates the good managers from the bad. Taking this away would make the game far too easy for everyone to manage GS and training.
Maybe it differentiates those with patience and time to waste on figuring out which rotations produce which types of minutes, etc. I probably fall in that category - dunno if that makes me a good manager.
For the purposes of game design, I can see why the BBs don't implement this.
I always thought of it as a cop out in game design. I would much prefer if the challenges involved real basketball team management issues like should I play my slow but tall 7 footer who can use his length to guard against the opponent's scoring center of similar build, or should I play a shorter big who is more nimble and can create a mismatch of his own on the offensive end. Etc.
But those elements don't exist, so I waste my time with trying to get exactly 48 mins for my 18 year olds, because any other number is just...bad management...