The main advantage is that you become more comfortable riding your own bike. If you have had a proper bike fit, your bike is ergonomically positioned to reduce stress, injury and fatigue on your knees, neck, wrists and shoulders. Only three adjustments can be made on spin bikes: seat height, handlebar height and distance from seat to handlebars. Training on your real bike through the winter makes you far more comfortable once you get back out on the road. And, if you have invested in a smooth, perfectly light, comfortable, carbon-fibre race specification beauty, you want to use it.