I miss the days when you could read science fiction and think, "none of this exists, I wonder what the future will end up looking like?" We are way past the conceptual stage for the future of transport, and I think it will look exactly like the mashup you are describing.
It could even be improved with printable self-driving cars, areas would have a base load of self-driving cars, but in case of need/rush the area center would just print new cars.
My biggest fear re. cars as a service is degradations, it's been an issue in at least some of the bicycle sharing systems.
That could actually be pretty cool. Basically, a ZipCar + Uber that doesn't suck. That would be the dream.