I don't see your logic...
All the weapons you buy in the tower are 331 so why don't they make the Crota weapons less appealing?
VOG, Crota & Vanguard weapons are all legendary - why should the VOG set not get any of the 331 love when all the raid weapons are basically exotics in disguise?
English
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Because of your last sentence that's why, because raid weps are a bump ahead of any other weapon in the game, because they have useful abilities while freeing up a slot for an exotic else where, it's a commodity that players should strive for, every raid, not simply handed because they have a crush on the previous raid weapons
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Tower weapons don't do any elemental damage, and they generally don't have the cool perks that the raid weapon do.