He's being let go because he doesn't shoot enough, doesn't drive the lane or play in the post enough, and passes all the time.
Sound familiar?
Don't we already have too many outside shooting PFs that overpass and don't play in the post?
We need a center. Diaw isn't one.
We don't have anyone off the bench who can facilitate an offense like Diaw can, and that includes our guards. He's on par with Pierce as a playmaker at the forward spots.
Having Diaw allows you to benefit from playing guys like Bradley and Pietrus together defensively without forcing them out of their skill ranges offensively.
When Bradley is in the game and playing PG, only 4 of the most used 5 man lineups with Bradley in them don't have Pierce or Garnett in them. Pierce and Garnett are in 8 of those top 20 5 man lineups with Bradley as the PG. There are plenty of facilitators on the floor when Bradley is out there as the PG.
We don't need more facilitators on offense. We need people who can score and possible create their own offense by going to the basket with the ball or playing in the post.
THAT is what this team lacks as a whole but most especially when Rondo is off the floor.
KG is less of a playmaker than a finisher at this point. He can do a little out of the post, but he doesn't make up for our back-up guards deficiencies; he's certainly not going to initiate the offense. Unless he's extremely aggressive, the offense tends to die whenever Rondo & Pierce are off the floor.
This is one of the main reasons why Doc plays Dooling, despite his deficiencies. He wants someone on the court who can handle the ball, shoot a little and try to get away with not playing Rondo and Pierce 40+ minutes per game.
The C's have just enough guys who can make shots off the bench: Pietrus, Wilcox, even Dooling or Bradley on occasion, can knock down looks that are created for them. That goes for KG and Ray when they see time with bench as well.
What they lack is someone other than Pierce and Rondo who can facilitate at a high level. Diaw can do that.