From what I understand...this is what I think...
Ainge made the trade so he could get Green, and later resign him. He would hope Green would develop into a potential all-star player, and keep the Celtics at least a playoff team compared to when they were in the dog days for 15-20 some years. If he had kept Perkins, he would have walked at the end of the season, unless he got paid. Even then, the Big 3 would be a year older, and well...sooner than later, they would have no one around Rondo.
With that being said...you DO NOT wreck a team's chemistry, especially if they were one quarter away from winning the Finals. I mean look, the Lakers didn't pull the trigger on Bynum for Melo, and look how that's working out for them.