When assessing any player on a very unselfish team like the Celtics, you have to look past the individual stats and take the team as a whole. The C's believe strongly in the "Team" concept on both ends of the floor, and in such an environment, no single player is going to have a big stat line, whether it's scoring or defense.
However, Paul Pierce is a very unusual player in that he goes about his job quietly, (most of the time), and contributes those intangibles that strengthen the team as a whole, but don't bring a lot of attention to him personally. Paul is a great defensive player, and at the core of what drives this team at both ends.