Well, each pop counts in a close game. With trainer lvl 5 or lower you might not even finish a player, especially a SF or PF. These days players receive a lot of out-of-position training to get secondary skills and since height and age seriously hamper the training, Lvl 6 or 7 trainer is a must.
If your player reaches desirable skills quicker, you get more playing time from him as a finished player and you can also start with another trainee sooner.
So those players with "only" 250k as a HoF might be capped anyway because of secondary skills that don't add up to salary.
Take a look at two of my players, Anžic and Navršnik. One has 13k salary, the other 123k. But there's only 3 skillpoints of difference between them.