That is very interesting option for someone who likes both cast and spoked wheels.
However, as ex car mechanic, I will have to disagree with your option only because of one thing. Your brake pads and brake disks are not perfectly smooth. The disks have some grooves on their surface. Those grooves are created by use and are replicated to brake pads. So, that way, the surface of the disk is perfectly replicated to the pad. You know, when you service your brakes without changing brake pads, you have to be careful to put back the pads in the same place the were taken off from. Or, when you put new pads on old disks you're told by mechanic to take it easy for first 100 miles as new pads need to "bed in". That is the time the surface of the disk takes to replicate itself to the surface of the pad. Only after it's been done, you achieve 100% of braking performance.
Now, after saying that, the perfect solution would be to swap your wheels but use the same brake disks in the same place. But that is too much hassle, I know, so the other set of wheels has its own disks probably. So, when you swap the wheels, all the process of bedding in has to happen, every time.