I have two bikes, my Trek 520 for three-season riding and my Specialized Expedition for winter commuting. Around here, winter is a very different beast from the other seasons and requires a different approach to riding. I suppose I could modify my Trek to be usable in winter, but winters are hard enough on bikes that I am happy to shield it from the winter elements.
The things that make the Expedition good for winter are that it has 2-inch studded snow tires; platform pedals that I can use while wearing boots and YakTrax; and has a straight handlebar with a twisting shifter so I can operate both brakes and shifter while wearing lobster claw-style thick mittens. OTOH, the Expedition is no fun to ride, and I don't ride any more than I have to during the winter. So, come spring, I get out the Trek and add joy rides to my cycling diet.