I agree with intent, but tying it to heavy armor has a number of problems. Most noticeably, nearly all casters can run around in heavy armor + tower shield with minor loss. Meanwhile, all light melee, and even some melee meant to be tanky like Mountain Stance monks, would be screwed. One either need to find fixes for these issues, or tie it to some other mechanic. I suggested earlier that they give increased %DR while swinging a weapon or actively blocking. That means they aren't casting spells or shooting stuff. Optionally they can increase DR when not shooting or casting, but that will let DC or burst nukers have it up most of the time.