I love KG, but if the Celtics are rebuilding, I'd rather watch him play for a championship elsewhere. No sense in keeping him around if all he's doing is taking minutes that could go to a young player who needs to develop.
I feel the same way about Ray and Paul.
I'd at least hang on to them until you actually have a young player that needs to develop. In KG's case, you can still theoretically shift his minutes to center if you feel he is blocking Johnson. I don't think Stiemsma is a talent worthy of being in a team's long-term plans, so I wouldn't be concerned about Garnett taking minutes from him.
If the Celtics don't acquire big contract players, I'd rather sign someone like Garnett rather than Kwame Brown to a one-year $6-7m contract.
I'd rather give playing time to, say, a group of the following:
Bass
JJJ
Stiemsma
Mahinmi (UFA center with upside)
Rookie PF / C
D-league call-up / young castoff (e.g. Tolliver / Randolph)
Green (starting at 3 but playing some at 4)
Is that a bad rotation? Absolutely! But that's fine -- even preferable -- if we're rebuilding. What you want is to give those players the opportunity to prove their strengths and work on their weaknesses.
I'd even be okay with giving playing time to a veteran (closer to 30 than 35), as long as they are a contributor we could potentially keep around for multiple years -- Wilcox, for example. But giving big minutes to a one-year at a time guy like KG at the end of his career makes no sense to me.