Pretty simple explanation, aoe dr is required for class diversity and the ability to clear content in endgame raiding. The thing that gives it away is that AoE heavy fights like Apex or pretty much anything in Gods have not been cleared on nim, and barely anyone cleared it on HM. I don't understand how Bioware could mess with raid balance this much, classes that do not have AoE dr (excluding healers) are pretty much unplayable in any NiM raid with strong AoE damage going out, Sorcs aren't even played anymore and neither are other specs without AoE dr. It wouldn't fix all problems in raiding, but at least it would be a step into the right direction to roll out AoE DR on all classes and to balance it around that instead of having most endgame content unplayable.