Another couple of caveats: Must have snowy winters and enough disposable income that you could have two bikes if you wanted.
It was the commuting by MTB thread that made me think about this. A couple of people made the observation that if you only have a single bike it makes sense to have a MBT, since they're more versatile than a road bike.
In my case, I have a road bike that I use about 8 months of the year and a MTB with drops that I use in the winter. I did use a road bike one winter but it only had clearance for the skinniest of studded tires which I found inadequate.
Giving it further thought, every person I know that commutes even once in awhile during the winter has two or more bikes. One of them does use a road bike year round (Crosscheck), but has an old beat up Schwinn he uses as a backup.
If you do only have one bike, what type is it? (road, hybrid, cruiser, MTB, etc).