Chris Forsberg of ESPN Boston beat me to the punch on this one.  Remember when Pierce signed and there was some confusion over that final (4th) year being fully guaranteed or not?  It was reported at the time that it was fully guaranteed and that's why the deal got done.  In reality it is a little more complex than that.

Pierce's future - Boston Celtics Blog - ESPN Boston

According to the supremely helpful salary database at ShamSports.com, Pierce's four-year contract is set to be paid out in escalating increments of $13.9 million next season, $15.3 million in 2011-12, and $16.8 million in 2012-13 before dipping back to $15.3 million in 2013-14.

The final season is only $4 million guaranteed, becoming $5 million if Pierce participates in 50 percent of Boston's games (regular season and playoffs) over the preceding three seasons, according to a note on the site. The deal is fully guaranteed if Pierce is not waived on or before June 30, 2013. 

The upshot is that if Paul's play declines to the point where the team is comfortable waiving him before the end of June 2013, then the team would only be on the hook for $5M (less if he misses significant time).

Now don't get me wrong, the Paul Pierce fan in me wants to see him stick around and earn his full paycheck.  But the fan-for-life in me wants to see the team maintain cap flexibility as we enter Rondo's prime.  Especially when we don't know what the new CBA is going to look like.  So I consider this to be a pretty nice win for Danny Ainge.

Pierce himself is talking about the end of his career being "three or four" years away.  So there's a chance that he could finish out 3 years and say "you know what, its been great, but I'll just hang 'em up and take $5M to walk away."  He's already talking about finishing his career with a year or two overseas so he could recoup some if not all of the money he's walking away from.

Regardless, he gets to walk away from the game on his own terms and the team has a little bit of flexibility without alienating the face of the franchise.  Sounds like a win-win to me.