I ride a longer route on snowy days for all the reasons the OP states. My summer (direct) route is 8.5 miles. I rarely use this route when winter is in effect with reduced daylight and snow/ice everywhere.

My winter routes range from 9 to 13.5 miles. The 13.5 miles keeps me off nearly every road; the couple short stretches of roads are slow residential streets. When needed, I will ride the shorter route in the snow, but it's just a lot more stress. I have prefect control in these conditions, but the cars, not so much. Drivers do the DUMBEST things on the snow.

Unplowed trails don't bother me, now that I have a Pugsley. But even in the past, I'd ride my 26"x2" studded tires on unplowed trails with up to 4 inches of snow. Heck, I've even made a couple PM commutes with my 700x28 bike with up to 4 inches of fresh snow (those days when it started gorgeous & sunny and I thought the forecast was lying to me about snow in the afternoon... and it wasn't).

I enjoy snowy rides, taking the longer, quieter route is enjoyably.