I'm divided on this subject. The Grimoire behind Gjallarhorn tells us just how rare it should be. But it would be unfair if everyone didn't have the chance to get one. I have four myself, the first one I got out of a legendary engram the week after Xur first sold it (All who have been playing since the start know how incredibly amazing it is for that to even have happened). The second out of the Vault of Glass chest and the last two off of beating Crota. It is a wonderful weapon but past events do not bode well its future.