Having both omen and reverie makes your ultimate a lot less situational; reverie lets you maneuver around real fast in case you don't like where you ended up, and urgot's ult adds 260 resists for a few seconds which cranks up the duration on the randuin active. I consider manamune urgot's only absolute must-have item, and between manamune and reverie I'm pretty good on mana, so it just seems to make more sense as my CD/armor solution.