Now that it has been clarified that a 700c front rim and tire and a 650b rear rim and tire can be so close in diameter that there should be no change in the bikes handling characteristics what is the purpose of using different wheels sizes?
My reason is to have a narrower tire in the front to achieve the type of steering control I prefer while using a wider higher volume one in the rear for increased traction and compliance while the overall diameters are very close to the same as I have stated here. Unless this is the reason for dissimilar rim sizes I don't see any need to do so and doubt it will gain any sort of wide acceptance. Especially for bikes used primarily on road surfaces.