Gear progression, abilities, talent points, quests.... You want to argue it's not an RPG really?
TTK, plays a role but i rather have my health constantly thrown down so that a fight can't last endlessly and i can't kite mobs just endlessly either. Taking damage from white hits is a soft enrage and that's better than merely having a timer in which you need to kill it in how i see it.
Also don't you think that if one aspect of the game had no white damage and all the rest did it comes across as inconsistent game design and confusing? It does for me.