I think the reason is because it would become a nickel and dime thing. Once people got the game mode added, then they would ask for rewards, then they would ask for XP to count. Basically, no matter what Bungie gave there would be the "next thing they wanted" right behind it. I agree though, it would be hella fun.
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Bungie can just say no, they've had no problem doing it with other aspects of Destiny.
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I think they already did ;)
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