Trading Perk in lieu of giving him a $6M - $8M/year extension also allows them to do a couple other things. If they need to go out to a third year to re-sign Ray Allen, they could do that and still have some cap space left in 2012. More realistically, they could give Pierce an extension so he can finish his career in Boston, AND probably have enough cap space to make a splash that same summer (depending in part on things like whether they spend the MLE this summer, and for how long).
Trading Perk might allow Pierce to retire a Celtic, keep Rondo, AND to pursue someone very interesting (*cough cough DURANT*) in 2012.