You can put Perkins on Dwight Howard for 30 minutes, not have to double team, and not get torched for 40 points. I'm not sure there are many other guys in the league that you can say that for. He can also rebound at will against Miami's front court, neutralize Carlos Boozer, and whoop whoever the Knicks have next to Amare up front. He's also the guy who guards Bynum when he's in and shoves Gasol around when Bynum sits. He's also the guy who would keep Blair and McDyess down while KG guards Duncan.
Shaq simply isn't that guy. Jermaine O'Neal probably won't be playing again this season. Troy Murphy or any other buy out guy can't do what Perk does. I have no clue what so ever what Danny Ainge was thinking with this deal. Neither do Garnett and Paul Pierce. At times, the Perk looks good because of the overall defense. Other times, the over all defense looks good because of Perk. He's much more than some 15 PER guy.
Additionally, the window is closing. The Celtics have this year and next year before its all over. Perkins was a big part of this group and the deal just does not compute.