Because not everyone will use a character with solar grenades.
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Hunters have trip mine nades, could be used for the same thing, don't care about titans but regardless, seems pretty stupid to waste your exotic on something for "area denial" I don't need to deny any areas if everything is dead
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Easier said then done when your defending a sync plate by yourself and facing down a charging praetorian.
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Every class has an AOE nade.