In order of importance, here are the reasons:
1) Consolidating trade exceptions, which could potentially be useful on draft night.
2) Prince didn't want to be here. He was a good guy while he was, but he wanted to be elsewhere. Reputation is important in the league, and trading Prince to Detroit is a good way to maintain that positive reputation down the road. Players and agents need to trust GM's, as we've seen what happened in Phoenix when that trust was broken.
3) Because of reason 2, the alternative was buying Prince out. Many believed that would occur. Certainly some teams probably held off negotiating with Ainge because they thought Prince was going to otherwise be bought out. Next time such a situation occurs, Danny has one more thing to point to and say, "No, I won't have to buy him out. Believe me, and let's make a deal."
4) Jonas Jerebko is mildly interesting, especially in the offense Stevens is trying to run.
Unlike some in this thread, I don't believe Datome is long for this roster. Yes, Ainge was interested two years ago, but you can't ignore the complete lack of production Datome has shown at the NBA level.