I think just looking at Posey as defense vs. Maggette as offense is really ignoring the whole argument. I think we also have to look at both players from an offensive perspective. I think if you really consider this from a basketball perspective, rather than from a stats perspective, you'll see that Posey is the better fit not only defensively, but also offensively.
The biggest example of why this is so is what we saw play out between Eddie House and Sam Cassell in the playoffs. From a talent perspective, Cassell is vastly more talented than House. He has a better handle, can create his offense better, and knows the little tricks of the trade to get to the line. House on the other hand--offensively speaking--only beats out Cassell with his shooting ability. Why then did House generally "outplay" Cassell throughout the playoffs?
Quite frankly, the Celtics' offense didn't need all those extra things Cassell could do. In fact, they were counterproductive. And they'll need those things from Maggette even less.
As flawed as Cassell was, his strengths over House did occasionally help, and probably would've helped had he played better. But the two reasons they mattered--the fact that he was a point guard and the fact that he was playing limited minutes--don't apply to Maggette. Cassell's skill set was important because he was a point guard and needed to handle the ball. That doesn't apply to Maggette, who will more than likely be playing on the weakside, yielding to the Big Three. Furthermore, most of Cassell's minutes usually came in the second quarter with either entirely a bench lineup or with one member of the Big Three. While I don't approve of the way he did it, at least in theory, he was supposed to be creating some of his own offense. While Maggette might would get opportunities like these, since he'll be playing more than the 10 or so minutes Cassell did, he'll have to play most of his minutes with 2-3 members of the Big Three. And he won't have an opportunity to create much of his own offense.
I think a lot of people who think Maggette will so seamlessly fit in offensively didn't closely watch what Ray Allen did this season. He probably made the biggest adjustment of the Big Three to come here. While all three of our stars gave up shots, he gave up more of a central role in plays, instead frequently becoming a weakside spot up shooter. And he struggled with that role at times. In his career he was used to being a central player in the offense, not sitting on the perimeter waiting for someone to kick out a pass for an open three.
My question is, if Ray Allen-arguably the best shooter in the league and a better player than Maggette--is going to struggle with this, what is Corey Maggette--an historically mediocre shooter--going to do in this role?
James Posey, on the other hand, has thrived in the role of weakside knock down shooter. He doesn't need to get plays run for him to make a difference. Furthermore, it's not just about man to man defense with him. I'd agree that in the playoffs Pierce and Allen probably both outplayed him in that department. But he's a phenomenal team defender who has a knack for the big shot, the big stop, or the big rebound.
Could Maggette come here, change his game, and prove me wrong? Sure. But I don't think there's any reason to believe he can do it besides the argument that he simply could. He's used to being a central part of the offense. I find it a little tough to believe that he's going to give up starting, money, shots, and the way he plays in general without something going wrong. For all the credit the Big Three get, none of them gave up starting or any cash, and in my opinion, Allen was the only one who significantly changed his game.