In my experience with only a rear disc in the city is that they work well, but the salt slush will cause the parts with threads to seize, the brakes pads get caked with rust though still work, & the action becoming stiffer maybe because of water getting in the longer run casing. My rear brake (BB7) is a bit unusual though as it mounts upside down with an adapter as my frame wasn't design with disc bosses so alot of crap ends up collecting in there. A normal disc above the axle will probably fare better & regular cleaning after rides which I often neglect will likely negate any of the above problems.

One of the main reasons I would go for it other then icing on the rim is the wearing down of the rim through such conditions.