The Pacers are desperately in need of bench scoring.  Terry may thrive with a change of scenery (look at what Jamal Crawford did this season after being awful with the Blazers).  Ian Mahinmi is signed for 3 more seasons at close to the same amount as Terry, while Terry only has a couple left.  Mahinmi was signed to be a solid backup center for the Pacers but he's lost his spot to Jeff Pendergraph and hasn't really played so far in the playoffs.

I think Mahinmi is somebody who could have a shot to carve out a role on the Celtics.  He's a young player who still has some potential if he finds the right situation.  This trade would save the Pacers money long term and perhaps make them better.  I wouldn't be at all surprised if Terry looks less cooked elsewhere.