If we talk tactically, snubbing Del Piero would be the most stupid thing Donadoni would do in his football career. Looking at how Totti retired and looking at the players he's called for the Spain test. There's no single player able to make a diff on the pitch and have the Dp/Totti style.
Actually, Del Piero is not being picked especially because of the tactics. (or that's what the papers in Italy think).
Donadoni prefers the 4-3-2-1 formation with two attacking players on the wings (Camo-Di Natale, or Iaquinta-Di Natale, or even Camo-Palladino or Quagliarella-Palladino) and there is no room for Ale in this formation.
Ale also made it clear that he doesn't want to play in this position.
That's clear and it's understandable why there is no place for Del Piero.
But there is one small detail that Donadoni must have in mind, and that's : Formations change depending on the opponent and depending on the result.
He won't play 4-3-3 all the time, he'll be stupid if he does so. That's exactly why he must have someone who can play as a support striker and also play the trequartista role.
Since Totti retired, he won't have a better player for this than Del Piero.
And as Rab said, since Totti is not available, no-one from the available ones is more influental than Del Piero.