But that's not true - defense is simply a rating for overall defense. I understand that height is factored in, but based on that, you could have a 6'0" guard effectively guard a 7-footer in the low post. That doesn't happen, sorry.
And come on, that does not happen all the time. Paul Harris didn't guard the post that often in the 2-3, and when he did, he wasn't guarding Dejuan Blair. 6'3" SGs don't guard 6'9" players in the post "all the time."