You did make several valid points, and I have no doubt that many people would certainly abuse the Gjallarhorn if he sold it. With that being said, I get sick of all these people thinking they are better than me because they bought Gjallarhorn when he sold it or got it in a drop. Suddenly, I must be a scrub because RNG has not granted me the "Special Wonder Weapon." Would I like Xur to sell it? Sure! For the same reason I want him to sell Icebreaker. If people want it, they will buy it. They will either use it or they won't. Personally, I prefer exotic Specials or Primaries over exotic heavies. So if I did get it, I would probably just use it for boss battles, or to allow me to fulfill different roles in a fireteam, like redabishai said a few comments down. The gun is easily the best launcher in the game bar none, so don't be surprised if they use the dreaded Nerf gun on it, or at least improve other launchers like Dragon's Breath to be more effective compared to it.