LBJ and Wade are plus defenders, but bigs are simply more important in team defensive schemes.
I disagree with Wade being a good defender. He has the athletic ability and physical tools to be a lockdown defender and he'll occasionally make a highlight steal or block which makes you believe that he is. However, when you watch him play defense on a regular basis, especially off the ball, you'll see a player that takes horrible angles chasing a player through screens, doesn't fight through screens, gambles way too often, and gets beat off the dribble much more than you would expect from someone with his athletic ability.
Perhaps this was due to him having to shoulder the load offensively and he might improve that area with help. Regardless, until I see it on a consistent basis I'll refrain from considering a good defensive player.
