For me and most of the veteran players Gjallarhorn is the only weapon we need from the game. I even got nechrocasm and everything else. The main reason for playing the game is to get it. If bungie makes xur sell gjally there would be nothing else to get, thus i won't have a reason to keep playing the pve content. Selling gjally would make us less interested finishing raids again and again so that would be a bad move Bungie.