There are three simple questions to asking yourself whether or not you require a trainer much past advanced:
1) Does my trainee have a high (MVP, HOF or ATG) potential, or is he merely at the superstar level or under?
2) Do i fancy trying to shoot for the u21 squad with my trainee (one could make this argument to the NT as well, but it would seem that few go this far).
3) Do i have a trainee who is significantly behind in pops for his age (applies mostly to 20-21 year old prospects with ~47-52 attributes)?
If you cant fit your guy into at least one of these categories, it is a waste of money. I have a level 5 because i have two prospects who fit into at least two categories on this list, and even still i find myself feeling as if it's not worth it to me to go past a level 4.