They do not feel like they should take up an exotic slot; and their jobs can easily be done by many other things in the game.
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They should give them an exotic move since they are specific to a character. Maybe the aerial move will work differently with the glaive. Hunter - when slamming down with glaive, it sends a circular Shockwave killing ads. Warlock - dashes forward slicing enemies in front like a samurai dash. Titan - swings like a baseball bat smashing enemy if front. Does more damage than warlocks and doesn't cover the distance.