I think it all depends on what winter is like in your area.Here in Ontario,Canada a road bike would be out of the question(except for the rare days the roads are clear and dry)A mountain bike comes with "snow tires" already.Sure there's a lot more road friction from the tires but if you're an avid cyclist you should have the legs to push the extra weight.Just a little slower.With me it's also economics,generic mountain bikes are everywhere for free.They're rugged and if rebuilt properly will last a few winters.I just keep swapping the good parts to the "new"bike.
I ride a pos mountain bike I found in the trash this year.I converted it with cruiser(upright)handlebars to get some of the weight off of the front wheel because I find most crashes occur when the front wheel slides out on you.With a lighter front end it seems easier to recover.I usually wipe out at least 4-5 times a winter.Nothing serious,I wear elbow and knee pads,my old knees and elbows can't take it anymore.I've been winter commuting for 13 years).Since switching to the upright style bars I haven't crashed yet,last 3 winters(knock wood).It doesn't "go down" so quickly when you hit those frozen ruts in the ice.
So,it all boils down to what winter deals you.If you only get a dusting of snow every now and then and only have to worry about the cold, then a road bike might make sense.There's a foot of snow on the ground and we're expecting a big blast tonight(maybe another foot )There's no way I'd take my road bike out this winter.Not yet anyway