First it shows me that this person probably doesn't use the rear view mirror ever. That's pretty dangerous in and of itself. Second how can you be sure there's no ice underneath that snow if you didn't try to brush it off? Third it doesn't take much to make it so the driver behind you can't see. Even if you're following an appropriate distance behind a chunk of ice or a flurry of snow can make it difficult for you to see the road ahead of you. Fourth it shows that the person driving this car doesn't really care about anyone other than themselves and are too lazy to take a minute and a half to brush the snow off their car.