I am extremely conflicted to whether I would like Marbury in a Celtics uniform so I've decided to lay out some Pros and Cons.
Pros:
-He would immediately bolster the offense on the second unit and give us a solid 6th man.
-He'd arguably be the best ball handler on the team.
-He can definently run the point better than Cassell and House.
-He seems to want to accept a lesser role, perhaps the pressure of playing for the hometown team got to him, plus the Knicks' problems. Possibly taking him out of the spotlight and placing him in a winning situation may help him recover his passion/skills/fire.
-Parallels to Randy Moss are inevitable, but Marbury's reputation is a couple degrees worse.
-We can sign him to a Darius Miles type contract and cut him at the first sign of trouble.
-He will have some strong minded veterans around him.
-He has an opportunity to rejuvenate his career and turn around some of the negative connotations, you have to believe a rational person would want this badly. (However, seeing some past interviews, it's not clear if he is a rational or emotionally stable person.)
-He has done some good things for the community.
-New Yorkers hate him. (He must be doing something right.

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Cons:
-The biggie: Team chemistry when you've got disruptions, they will be an issue, especially with Doc (Recall, Marcus Banks, Ricky Davis, Mark Blount, Marbury is arguably worse then all three combined.)
-DUI (I wasn't too pleased originally with Pose signing because of this but I'll be willing to overlook it), and involvement in sexual harassment and sleeping with an intern does not exactly scream character guy. Keep him away from the Celtics Dancers.
-With Miles and Marbury on the same team, it certainly increases the chances of distractions.
-Each and every team he leaves, gets drastically better.
-Long history of coach-killing.
-Is he ready to accept a supporting role?
Overall, I'm not sure the pros outweigh the cons, but again, if we sign him to a low risk high reward type contract it might work.