Would you buy Etheric light from Xûr or grind for it?
Edit: Remember, Xûr's strange coins are losing purpose cuz we've all gained exotics he sells
Edit 2: Once a week
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If he sells them it should be rare like once in 1-2 months and even then it should be expensive like 40-60 coins (if it is so rare that you eventually get to drop only one from Nightfall, who knows for sure)