The problem with Xûr selling Gjallarhorn is that PvE content would become trivial immediately. Prison of Elders bosses and their mechanics, outside of Skolas, can and will be completely ignored. Same with Strike bosses, even though they're just bullet sponges anyway. This isn't a call for a nerf, but I'm reserved about him selling it again. That's not to say I don't want people to have one. I got my second one from Crota just now, with my friend (relatively new player) who doesn't have any exotics outside of Xûr, the guaranteed PoE chest from your first run and one Nightfall. I felt bad it was me and not him. Doesn't help that I got 2 Last Words in a row from Atheon and Ir Yût, and Fatebringer and Vision of Confluence, and he got nothing. This game, man.