I am not a fan of trading Pierce and Garnett, but if I were, it would be because the team is in a situation where I would be willing to trade Pierce for a bad contract with 2-3 years left and a first-round draft pick.
The most likely team to target would be a non-contender that wants to dump salary, since Pierce has a partially guaranteed final year (but if Pierce were waived by his new team, he couldn't rejoin the Celtics until a year after the trade or until his contract would have ended, whichever is earlier), especially to avoid the luxury tax.
Imagine, for example, that the cheapskate Bulls offer Carlos Boozer and a lottery-protected first-round pick for the right to waive Paul Pierce and avoid the luxury tax. Do you take it? Do you ask for two picks?
What sort of compensation would you require to take back Bargnani (and perhaps Linas Kleiza) from the Raptors?
Can you accept trading Pierce for a contract that makes it unlikely the Celtics can be a contender for at least a couple of years?