People point to bench scoring as an area that needs to improve, yet I am unconvinced that the Celtics should target some of the names being tossed around who can be described as all-offense, no-defense players. (Some people have suggested Jamal Crawford, for example.)
Volume scoring tends to be an overrated skill, so we're looking at players who tend to be overpaid. And if this player is going to be a bench, that suggests the possibility either his scoring ability is not good enough or his defense is too atrocious to win a regular starting job in the NBA. Of course, he may just be too injury-prone or a veteran ring hunter who does nothing to help this team's lack of athleticism.
A player with a reputation for bad defense may be a bad fit. There are different sorts of poor defenders. There are those who just never seem to know what they are doing. There are inconsistent defenders who sometimes seem to have a clue, but perhaps lack effort. The latter may be fixable, but a bad defender just may be too incongruous to the team's defensive identity to work out very well.
The bench offense's true need may be a better playmaker instead of a scorer who can create his own shot. Ainge's apparent obsession with combo guards has kept the Celtics from developing a pure point guard who doesn't play shooting guard. While some have made the argument that Rondo is young and can play extended minutes, so a backup point guard is an unnecessary luxury when there are more pressing needs, I suggest that a dedicated point guard backup would allow Rondo to play all-out instead of having to reserve energy and not push the ball as often as he could. Plus, the team could be more willing to give him a game off if dinged up.