Perhaps it's the preference or philosophy of my cube playgroup, but when they see three colour cards, it appears they want payoff cards, not build-arounds; for the latter, they're more than happy to take them in mono-colour.
I would argue that Jeskai Ascendancy is the payoff card. All you have to do is throw together a spells matter deck, with a hint of either ramp or tokens or even weenies/counterburn. Ascendancy generates so much value in any UWR deck, its practically goodstuff in the right environment. Instants turn into hand fixing combat tricks of doom? What UWR shell doesn't want that. It also happens to be a build around and a combo machine.