Marion has come up small in the playoffs the last three years, so they decided to take a chance and do something different without giving up their best two players. This has a high potential to backfire, but if Amare shines back in his old position of PF, I think losing Marion is a wash.
So offensively there is some merit to the trade (Shaq drawing doubles low block and kicking out to shooters). But it's the defensive downgrade that Kerr fails to fully understand. The formula against Shaq will be simple, pick and role him to death. Seriously, who here can possibly think a Nash/Shaq duo can defend against well run pick and rolls by Williams and Boozer, Terry/Devin and Dirk, Kobe and Bynum/Gasol, Parker/Manu and Duncan? Lambs to the slaughter.
The fact that Shaq can't stay on the floor physically (stamina) and because he's foul-prone at this stage of his career is a huge downer that no amount of motivation from being shipped to a new city/team will make up for.
Here are sum reaction from the players(following a double OT loss versus NO, probably the best game I've seen this season):