I agree with giving some boss mobs (and even some trash mobs) ways to mitigate defense. Boss mobs should have more than one, trash shouldn't have more than one type of defense that they ignore.
Another possible idea for tanks would be to implement a shield block (not the garbage we have now) where you have a % chance to actually block the attack and take no damage. This percentage could be improved with gear, or perhaps even tied to some skill. Sword and board should be competitive in regard to DPS and way ahead in survival.
It just ain't so.