TJSpiel says:
Personally if I could have a single bike that worked well for all my cycling activities then I would have only one. It's much simpler to only worry about one set of lights, tools, and accessories.
If my bike has a flat I fix it. I also do all my own work so there's no worry about having it sit at an LBS for days. If my bike were out of commission for a day or two, I have the same backup I use for when there's 4 inches or more of unplowed snow on the ground. My feet and the train.
Much of what you say here is quite true TJ, but what if you awaken late and have a flat? ...Having a spare bike can be quite handy. Also, the overwhelming majority of us cyclists are not that mechanically inclined. Therefore, most of us are at the mercy of the LBS mechanic. Traveling by foot and by train are both wonderful alternatives to commuting by bike. However, to many us, cycling would remain a preferred method of commuting.
That being said, I do have a winter bike. The bike I like to ride when the roads are good doesn't have enough clearance for studded tires.
Ahh...and there goes the dreaded N+1 rule again. It's practically inescapable!
My winter bike is actually newer and more expensive, - largely due to the disc brakes and gear hub. Road salt and grit are tough on the components.
Ahh...and yes, my friend! That's why having a single speed for the windy city's most nefariously inclement winter days, would be best suited for just such an affair...
Besides, Chicago is mostly flat anyways...