I think they should be build in close counselship with the player´s owning team. If the team is willing to develop and to maintain a high salary player and is committed to the guy reaching NT stage, that player can and shall be trained for the optimum, which means for further NT use.
If a team is interested in "short term" benefits, and plans to sell the player after he´s out of U21 range, I´d encourage the coach to build a useful player for U21 use while keeping in mind what he -may- be worth if he´s still NT material when he´s leaving the team.
Key is keeping track of the players and their teams plans, so that the high proifle prospects don´t end up on free spending foreign teams unwilling to continue with the development instead of teams eager to support the NT campaign.
Zwei Dinge sind unendlich, die Dummheit und das All...