If anything, it's the only way to earn exotics. You have guaranteed ways to get the coins to buy the exotics, and getting exotics to drop is RNG. That being said, most of the exotics that xur sells are underpriced.
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Underpriced is why I don't consider them to be earned. I respect RNG drops way more than xur bought. RNG drops, albeit frustrating at times, on a larger scale represents time played getting your exotics. Xur is a cheap whore who comes around every weekend putting out for practically nothing.
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Yes, I believe that only earned guns are bounty guns, nechrochasm, and the vex mythoclast. If xur sold guns like the ice breaker for 40-55 ish coins, I would say that it was earned. As is, not so much.