The main reason for a 650B is that most road bikes have no room at all for larger 700C tires and even the few that do have no room at all for fenders. So when you move over to 650B on a road bike, you do two things: you reduce standover height, making the bike safer to ride and more stable and you gain room to install fenders on a road bike. In short, you can transform a road bike into a city type bike for not too much money. Once you do the conversion and the late Sheldon Brown wrote an article about 650B - you will not want to return to your old road bike configuration.