In my experience SS, C, and CF tire faster than 1B, LF, and certainly DH. If all of those players are 80 durability the C, SS, and CF go below 100 first. This tells me that the playing time being used by the engine is at least partially based on positioning. Harder position to field uses more durability to play. Unless of course I am missing some other variable that I am not taking into account.
Part of the equation on improvement in this game is based on "playing time" not position played I will grant you that. I just think that by playing a more difficult position you can get more credit for having "playing time" if you play SS rather than 1B or DH. Just as an example I will say the same player with 80 Dur might be able to play 80 games in a row at CF until he goes below 100%. If you put him at DH he might be able to play 100 games in a row before he goes below 100%. In those 80 games the player improved just as much as he would have by playing 100 games at DH.
The rub is most don't play prospects much below 100% for fear of injury. IF you played a guy at SS for 140 games he would see more improvement than if you played him at DH for 140 games. However in order to play a prospect with a Dur of 60 at SS for 140 games they would likely be down to about 50% by seasons end and the chance for injury (negating the extra progress) would be very large.