This is a tough call for me, as I'm sure it is for many Cs fans. I never want to see Paul Pierce in another jersey. Period. And it's true, like Nick said, that even a struggling Pierce is still shooting better than Gay, overall and from 3. Plus, Pierce is battle-tested and has come up big in the clutch several times, and we'll need that in order to win a title this season.
I also agree with others here that this type of trade doesn't address our biggest need (another legit big), or even our second-biggest need (a backup PG), so why bother?
If we can trade from our surplus of 4s and 2s (namely Bass and Lee) for a legit defensive center (Gortat, Dalembert, or even Mozgov), and also get a decent backup point, we'll be all set.