Snow would be cleared off before a regular circuit race, but only off the tarmac/kerbs and even then there's the possibility of wet areas or ice if it's still really cold. And of course, people who came off the track would have a much harder time getting back on in a couple of inches of snow than they would on regular grass.
And what if the snow was still falling? You can't have someone clearing the track while there's a race going on, so you would get the odd drift here and there, depending on the wind. So it definitely has a place, even outside of rallying