If you hold on to Pierce, there is a decent shot at the Celtics being able to get a better return using his deal as an expiring contract.
Imagine, for example, the Grizzlies wanting to clear cap space by offering the longer deal of either Zach Randolph or Marc Gasol for Pierce and Sullinger. Maybe Pierce to the Wizards for Nene and a draft pick. Or the Bulls determine that Derrick Rose is toast and decide to blow up their team, making Joachim Noah available.