I used to work in the tire industry so I have to throw in my 2c on this 'exciting' topic. You do have all-weather tires or more commonly known as all-seasons tires.
All seasons tires serve their purpose pretty well. However, they are not the greatest in dry, they are not the greatest in rain, and they are certainly not the greatest in snow.
I live in MN so the winters get pretty bad here. The plows do an excellent job so we rarely have to encounter driving in 10" of snow on the highway. But the side roads and city streets are different. They don't get plowed as fast. These all season tire 'should' do fine though because they are narrow.
My biggest fear is not the snow but the ice. This is the main cause of accidents in the winter. Expedition, Escalades, Wranglers, 4runners, Cherokees, Hummers-I've seen them all in the ditch. Why? When they encounter ice, 4x4 is totally worthless. They just can't brake in time.
That's why I plan to buy snow tires (like the Blizzaks). Trust me, when you're driving in January and the pavement has that 'dark-color' to it, generally, it means you're driving on black ice. That's the stuff that causes fender benders and people to go in the ditch. The snow tires will definitely help. There's no comparison between the Continentals that are on stock against the Blizzaks.
Trust me on this one folks. If you live in the northern hemisphere, you will need them. To stop your Smart 5 feet shorter than someone else in an emergency braking situation is priceless. Call it insurance. I know there are people that have all-season and will say they have gotten by OK with them. I just don't think they've driven on snow tires before.
It's a no-brainer IMHO. Do yourself a favor and get them mounted on some cheap steel wheels. I'm guessing Tire Rack should have a combo set for under $800 by this winter.