It's not about being top dps. Outside of static groups, it's about exclusion of melee (or a token melee) because they present more of a possible liability to the group than bringing another ranged damage dealer. Again there is nothing to go by with Leviathan yet but if (and I do mean if) he can't be flanked or attacked from the rear, melee damage suffers. The overall community will not want to take them. This is what the thread originator is afraid of happening.