I always thought Pierce was a good defender, he almost always outplays his counterpart on the other team.
Pierce is one of those elite players who doesn't get the respect he is entitled to, since the teams he's been on haven't done anything significant in the post season. He has always been the Celtics best player since he's been there, and even with Garnett and Allen on the team, he is still the best of the three.
I haven't watched him enough over his career to say if he's a top ten defender or not. But Boston is the best defensive team this year, and he's a big part of it, so that suggests he's pretty high up there.
At least you know he thinks defense is important and he works at it. For someone who is an elite scorer, he works very hard on both ends, he's right there with Kobe and AI in that regard.
