Bynum is a good interior defender, but their perimeter defenders are atrocious..Artest is still a good defender, but looks like he has lost a step(to be fair, he looked like this last season too, then he stepped up his D during the 2nd half of the year, IIRC)..Kobe is an average 1 on 1 defender now..Fisher/Blake are 2 of the worst defenders in the NBA..Shannon Brown is a waste of athleticism on the defensive end..
So Bynum is often put in foul trouble due to the incompetence of his perimeter defenders..when he is in the game, he obviously makes a difference on the defensive end..
I'll say this..if Gasol isn't coasting and the mileage of the last 3-4 years has gotten to him(also his rumored hamstring injury or whatever), then LA will be eliminated in the 2nd round, he's way too valuable..