Some would agree with you if there was an actually earn to get system (ie complete a crucible with the highest number of kills or with the best kd, have the highest number of kills in a strike , most attributed in a raid) then your complaint would be valid. Seeing as to how every thing is random the best way to EARN an exotic weapon is to EARN strange coins which is done by completing nightfalls so if you hed earned 27 coins by running the nightfall with 3 different characters than you deserve to spend those coins (which you earned) on what Xur has to offer.