As a Warlock, it's probably because us and Titans can push Atheon off at the end.
Warlocks also have self-res and Titans have the shield. Tends to be more useful support-wise than the pure offensive capabilities of the Hunters. Although in terms of the Atheon fight, I find Hunters very useful with their golden gun in taking out the enemies in the portals.
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What I find useful is blade dancer to get rid of the smaller add-ons or to revive downed teammates by going invisible with the super or crouch depending on scenario...granted on hard I'd only use invis to get the hell outta harms way XD