The issue is corrosion on the rotors, aluminum or stainless takes care of the issue. I think the softer aluminum would also provide better braking when needed, but since regen will handle the task most of the time you won't have excessive wear on the aluminum that you would with an ICE.
I don't know much about brakes but this sounds like a great idea. We live on a dirt road in the summer and they salt the paved roads in the winter so I end up washing my car a lot. The result is there is often a thin layer of rust which loads up the pads and I have the problem Doug_G is talking about. The trouble with his solution is I have to do it at least once a month to keep my brakes strong. I suspect that stainless rotors are avoided because they don't dissipate heat as well as steel. But who cares I never generate any heat and I could always switch for track days. Does anybody know where I might find stainless or aluminum rotors?