Thank you for your answer.
In my mind, the training in itself is not an issue, player still get trained, but rather the fact that only the U21 manager knows what he has, and can guide managers to shape a coherent roster. When Cletus contacted me, my first question was "what do you need, I can train him like this or like that". Without guidance, you can end up with 4 great SG but no decent PG, 5 well rounded SF but no real big players, etc.