I guess having mirrors on all the houses could cause problems for other people, even for flat roofs. But I agree: reflecting the incident beams is important: when it emerges as heat (IR) it can be trapped by GHGs.
I had been thinking of the reflectivity benefit rather than the reduction in air comditioning.
Specular reflection (a mirror) would be ideal for sending energy back out into space most of the time but would occasionally death-ray your neighbors. White paint isn't quite as good (some of the diffuse reflection will hit non-reflective things) but it also won't incinerate anything. Feels like a good compromise.
I had been thinking of the reflectivity benefit rather than the reduction in air comditioning.