Xur does not devalue exotics. It's the loot system itself. RNG does not factor in player skill or contribution. You can be dead last on the losing crucible team and still get an exotic, or join a nightfall team just as your teammates are dealing a killing blow and still get one. RNGesus spins a wheel and gives you a consolation prize just for showing up. Honestly, I don't experience any feeling of accomplishment and it feels more like a handout getting an exotic drop from a lucky drop than buying it from Xur