Thank you sir. I appreciate the response. I can tell you as one whose played your game for five years now or more, this has always been a problem. Whether I was playing as a tank, or playing as a support class trying to heal a tank. The damage is just absolutely wonkey, there's no predictability or consistency to it, you're either dead or alive, and when the game is in that kind of state, why go for anything other than damage? Kill them within 3 seconds before you die yourself. If you offer players incentives to need tanks, support, and make monsters reflect those needs (atleast as far as group content goes), then its at least reasonable to surmise that players will begin to fill those roles and group content will be more than just nuke everything down or die.
Either way, I wish you luck sir.