You say he ruins it. How exactly? You are allowed to pick your own guns and wreck havoc with them. If you can not kill someone it is most likely you that is not playing well, and not your opponents weapon. Now your argument is that people who use Xur are not earning the right to use an exotic weapon. Not everyone can play all the time, and grind hour upon hour. Xur allows these people and those who have no luck with RNG to purchase exotic items so that they may enjoy another aspect of the game, and enjoy the perks that they bring. At the end of the day, if you wish to do well in the Crucible, get used to people having strong weapon and armour loadouts, and find a way to win.