A player like Del Piero, you can't just judge on how many goals he scores. He contributes with much more then just goals. He is the captain, and I don't know any other player, who gets that amount of on-pitch time, or who has more experiance playing for Juventus.
He has gone through more difficult periodes in his career then most players, and still he manages to score only three less goals then a player like Adriano.
Besides Capello never liked short players like Del Piero or Montella.
(Sorry if I'm repeating somone, but these threads are getting kindda long to look through. If I am then just ignore this post.)
I think the bottom line concering Del Piero, is that he is just not capable of doing the things he could before his injury in 98 anymore. I heard Bangsbo say in an Interview, that him and Lippi tried to "reschool" him, by releaving him of some defensive duties. This is why I think some people find him as being invisible in some games. But most of the time, when he has the ball, he's actually not that bad.