that's true about their only being certain moments in time. also, there might not be a terribly large diversity of stamina right now... or the modifier for how quickly players get tired depending on their stamina might need to be turned up... but i'd have to look into that in detail.
however, because people were complaining that the coach was overriding their own decisions about who was better to be on the floor... for SFDC I took out the modifier that had to do with the players actual skill. I can't really guarantee that your backup is going to play less than your starter, or more than your reserve unless my formula for whose better exactly matches up with yours...
this however does have one bad side effect. Now when your depth chart is exhausted at one position. Say, your backup is playing a different position and your starter has fouled out and your reserve is injured or something... then who gets put in that position is basically random and has nothing to do with whether they are meant to play there.
just to give people a sense of how complicated all the factors that have to be taken into account... currently we consider.
how good each player is at each position
whether its a blowout.. we rest starters
how tired each player is
whether a player is in the game already (can't have people coming in and out at every chance)
can't sub out free throw shooters
what is the foul situation.. and this has to change as a function of the point in the game
some modifier based upon the depth chart and the substitution pattern selected
then even after you have all those factored in correctly, you aren't just dealing with one position, you are dealing with 5 and you cant' have someone play two positions at once....
so if you start thinking about it carefully, you'll realize how complicated and convoluted a purely logic based approach would be. So we've gone to try to quantify each of these factors and use them to modify some global number which stands for something like... at this moment in time, how much do i want this player in at this position.