Because it make training some skills harder than others. In my opinion all kind of training [without team training] should have 3 options : training for 1 positon/ 2 positions/ 3 positions. I want train young SG but JS training for 2 positions estop me to tap maximum minutes for young player. In my league i'm sure if i use two 18 y players i will lose 90 % games.
maybe it is abit offtopic, but i normally make this training as a plan B when the minutes for single Position training are a bit crappy.
In my team this works pretty good, and in this case i don't find it that bad that non optimal palyers get a bit training.