I would love to have Marion as Pierce's backup but it sounds like an impossible scenario.
First, Marion will want to be paid with the MLE, at least, and the C's probably won't use it all on a wing backup since it would prevent them from signing a good big man.
Second, I don't think that Boston is the first destination that Marion has in mind. I can understand your point that he isn't going to Cleveland (too much "egos" already) but why wouldn't he go to Orlando? He could even be in the Magic starting lineup if we assume that they don't resign Hedo! He would be in Florida, could party often, and live under the sun, both hobbies that Marion is keen on.
If, somehow, we could get Marion for almost nothing money wise I'd say it's a no brainer but unfortunately all elements point out that he isn't going to sign with the Celtics, and I'm sure Danny and Doc aren't going to give up other free agents just to give Marion the MLE. Though, he could be one of their first choices, maybe you're right, we'll see!