It's harder to earn a gun from Xur then it is any other way, you get 1 chance a week from Xur and 6 from weeklies, 12 from raids and not to mention the exotic drop rate of dragon strikes...
Yes this is assuming you have 3 level 32+ characters.
So is it easier or harder to get the weapon you want via Xur?
Xurning is earning.
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I agree with your sentiment, however, xur still has an element of luck to it. Imbsure I've missed plenty he's sold. He's also only optional, so people can chose wether to use him or not.