Rebounding is a part of defense, just like position, blocks, and steals. Let's take a player, we'll call him player x. Player x plays relatively bad defense but rebounds the ball. Now, let's say player x stops rebounding. He plays relatively bad defense and he does not rebound the ball. How would you characterize his overall defensive value, did it go up or down? It went down right, because rebounding is a part of defense. A player can still be a good defender but not a great rebounder, because there are other aspects of defense. However, a bad defender who rebounds the ball is better than a bad defender that does not rebound the ball. Why? Because rebounding is a part of defense.