Running a single speed or fixie in winter would be nuts IMO. My winter commute tends to have stronger and more variable winds than in summer. This really calls for gears.

Most critically - snow. I normally (in summer) run about 18 mph average. When I swap on the studs this drops to 12-14 mph. In 2 inches of snow? 8 mph. That's right, 2 inches of snow cuts my speed in half. I don't know how anyone can cover 8 mph to 14 mph with a single gear range and not damage their knees.