thank's Tom 4 ur comments

I go straight to the point:
As I reckon players like Zamby, DP, Vieri as outstanding players,
I'd reckon that in many matches, excellent players go through a sort of "lack of application". Nothing wrong with it, if you consider how many games a player must attend if he aspire to play all the Cl, Serie A and NT.
But considering that nowadays, neither on the Cl, nor on the Serie A, nor on the NT, there is a chance for an "easy match",
I would presuppose that one player can't afford to focus all the above competitions.
So that is why, giving the chance for NT to young but talented players, would be helpful for both specialist player (which could get a deserved rest) and for promising player (which could show their own skill on significant NT games)
As for Donati, I believe he played several times on the U21 as a left, so he shouldn't be a prob.