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.
My new gears... 48 and 16 for 74 gear inches, which I will probably lower when I build the new rear wheel.
This appears to have been spoken by someone who has never ridden a fixed gear in the winter...
I have ridden fixed for two winters, used a 3 speed last winter with my fixed gear, and rode ss and geared bikes before that... and I know what winter is.
By far, the fixed gear has been the superior machine as it allows one to have incredible control, is very efficient, and has lower upkeep.
The wind always blows here, we get snow a plenty, and my knees have never been better.
I rode my fixed touring bike / commuter 82 miles last Sunday and a good portion of the ride was not pretty.