It's not a case of trying to keep things unique or rare. They're just trying to stretch what end-game content they have as far as humanly possible since there is so little of it right now. Someday we'll have loot lockouts and myth tomes per job but I don't think it'll happen for 6+ months give or take. I am annoyed by this as it would be cool to be able to gear up my tank and my DPS so that I could utilize either depending on what my FC needed instead of filling in with pugs.