Having only one exotic armor/weapon is just a balance thing. With that rule in place they can make the exotics much more powerful and crazy.
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Just make the enemies harder ?
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I could see if this was for the crucible, but I mainly play strikes and there really isn't balance needed for those since that it's just a dps race
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Take then, for example, the helm of inmost light. The helm gives titans te of their subclasses talents. If they have that level of power to each exotic there wouldn't be any choice and titans would have all those options avaliable.