We don't have winter the way you do in St. Paul, but a single taste of it on one commute home prompted me to buy a fixed gear and enclose the drive train. Icing up on the cogs, both derailleurs, even the chain, then stopping in the storm to chip off enough to get going again, was too much to risk a repeat. I guess a veteran of MN winters would know to keep the chain moving no matter what, and never shift, and work the brake cables so they don't freeze, but I suspect those skills only come with a lot of hard winters.

I don't know that rust is as big an issue. Salt and grit maybe, but honestly our winters are probably consistently wetter than the icebound ones, and I just wipe it down keeping lube on the chain and it's alright. So I wouldn't consider that as a major factor. I know that grit from the snow and slush will accelerate wear though.