I can see a lot of reasons why Pierce would re-sign and a lot of reasons why he would bolt.
The biggest reason, in my mind, which would explain his departure, is that he can't accept the fact that Rondo is now the leader and star of this team, that the Celtics are now Rondo's team. Sounds like a dumb or absurd explanation but we all know Pierce's ego and we've all seen this season how this change of leadership could affect their play on the court (refusing to give the ball to Rondo on certain possessions, not playing really together...).
So, for that reason, I wouldn't be surprised to see Pierce take a max offer from the Clippers to go play for his hometown and join the (pretty good) core of Baron Davis/Gordon/Griffin/Kaman.