It looks like I'm in the minority on this one (lol) but i think the Cs could win another championship with a healthy and happy (big ifs) as a backup PG, assuming he gets bought out and signs for the vet minimum.
I know this guy has had his troubles in NY, but I also know he loves basketball. Most players get to a point in their careers and lives where winning a championship means more than anything else in the game (luckily the Cs had 15 of those guys on one team in one year).
Marbury, who will have made a tidy $150 million after the buyout, certainly won't be motivated by money in his next NBA stop. I think that the examples set by KG, Ray Allen, and even PJ Brown had to be noticed with envy by all the ringless vets looking at the last few years of their careers.
I think the Cs' locker room is strong enough to handle a strong personality and possible troublemaker like Marbury. As long as the guy still loves basketball, would be willing to take on a complimentary role, and really wants a ring, it could work out. Talent-wise, it's an brainer. Despite diminishing productivity and effectiveness the past few seasons, a (properly motivated) Marbury at the minimum is a steal.
I remember the last two times the Cs acquired a legendary NYC point guard with a me first persona and questionable career. The first was Tiny Archibald, who many thought was too selfish to be a Celtic. He did OK here. The second was Kenny Anderson, who acquitted himself quite well here too.
Now, if Marbury isn't 100% committed to playing in a backup role, all bets are off.