Let me define useless.
1: Any player with 1 ball potential. It doesn't matter if he's a 5'9 PG, 5 ball A+ 18 year old. If he has 1 ball potential. He will not train. And with A+ initial skill, he may reach his soft cap immediately, especially if his potential turns out to be announcer or bench warmer rather than role player.
2: Any player with less than 3 ball in skill level.
3: Any 19 year old with less than 5 ball, A initial skill and 4 ball potential. A 19 year old player would have to have 8, 9 or 10k initial salary just to match an 18 year old with one season of training.
Anything else is ok to train or sell.