there is a very fine art and high level of skill in managing game shape, training minutes, training strategy and winning games during the 3 games in the week.
For me, this skill, is one of the fundamental parts of the game and once you get good at it, allows you to become better than your opponents.
So my point is, is that once you understand all these concepts, it becomes easier and easier to make sure that your player gets the desired number of minutes in the week.
If there was a facility to make sure bench players get subbed on after X amount of minutes - then that makes the game too easy! If this was the case, there would be no need for game shape management, don't worry about the 48+ for training mins because everyone will get them, forget stamina, forget having to buy extra players during your cup run forget etc etc etc etc....
essentially you can see how many intricate game concepts are affected by this variable. The fact that, as you play the game, you get better and better at doing this, means that it is a reflection of your management skill and, this is a management game right?
So whilst it would be nice to have a magical button that says 'substitution now', I think you will agree, that, once you know how to manage your game minutes, you will become better than your opponents and will agree that having this variability in the game, is actually a good thing :)