While for the optimal damage rotations I can understand using up Embers ASAP, I can understand in certain situations where you would still be casting Incinerate and not Chaos Bolt (For example, certain PvP situations, mobs that are too small and "unimportant" for CB, etc,) not to mention levelling. I can forsee myself holding off for Chaos bolt a lot for full embers/spheres
As for floating, it would make more sense then the crown, plus that's how Kael did it