Well one one hand he's a career back up that had a great few years between 2006-2008 , the Spurs front office get what they paid for really. I think he has a good skill set in terms of passing, but I don't see that utilized enough, maybe more screens in the high post , more two man games with Parker, some how use Diaws vision, height, and passing at the top of the key to feed slashers or spot up 3s.
On the Defensive end, he's bright and crafty but he can be bullied by bigger , more physically imposing centers and forwards. he doesn't rebound as well, and is a liability on D in the post.
To that end I feel we are under utilizing Diaw and Mills, to an extent, but neither players have really impressed me (imho) so far. I will chalk it up to needing a ling regular season to learn the Spurs system, and being given the time to fit in and make mistakes now as opposed to April or not being used at all.