Colder weather and variable road conditions mean a change in wardrobe and riding style. One question you might be considering is: do I need a bike just for riding in winter? The short answer is: maybe? We'll go through the pros and cons of having a dedicated winter bike below.
Usually, it makes sense to go for a more comfortable and less sporty ride. However, ultimately, the decision depends on a lot of things, but don't forget the simple rule: the correct number of bikes to own = N+1 (n = current number of bikes owned).
Save your nice bike: Winter roads are covered in salt, ice, snow, and god-knows what else, and as you can imagine, that stuff can be pretty tough on drivetrains, especially lightweight derailleurs, cassettes, chains, and brakes. By using another bike, you'll keep that fancy stuff in primo condition for when the weather gets nice again. You increases the longevity of all your components by not wearing out the same parts everyday, all year long.
Swing a couple bats: Just like baseball players will swing two bats before they step up to the plate with just one, riding a heavy-duty winter bike will make keep you in shape, so when you straddle the nice bike again, you'll wonder why you ever felt so sluggish last spring.
Get comfortable: Being forced to take your time and go a little more slowly on your winter bike can remind you what cycling is all about. It can be refreshing to take it easy and enjoy the quiet snow covered streets. In addition, typical winter bike accoutrements like big tires, easier gearing, and a more relaxed frame geometry will make for a more comfortable ride.
C.R.E.A.M.: (cash rules everything around me) There's no way around it: getting another bike means spending money. An alternative would be to winterize a current bike with fenders, wider tires, and a good drivetrain cleaner.
S-1: (s= number of bikes that will cause your partner to leave you) For most of us, ample storage space is simply not a perk of city living, and if you live with others, one more bike might be a bridge too far. Of course, you can always just stick some pinheads on those wheels with a nice U-lock and leave it locked up outside!
AWOL: You might become so addicted to winter bikes that you abandon your friends and family and head to Alaska to bike the Iditarod.