Of course, some might say they don't need winter tires because the cars they drive are equipped with say, ABS. The presence of such additional safety systems on a car means nothing for the traction properties of the tire. The single component responsible with how a car performs on icy or snowy roads is the tire itself.
HOW MANY WINTER TIRES DO YOU NEED
Using only two winter tires and leaving the all-season, or even worse, summer tires on the other wheels will make the car perform in ways you never knew it can. For instance, winter tires on the front and non-winter on the rear may cause your car to drift while cornering. Understeering and oversteering may become real problems in case you chose this compromise solution.
WINTER vs. ALL-SEASON
All-season tires are however a compromise and, as any compromise, they don't offer the best solution for both problems, but only a partial solution. Better than summer tires on snow, they are however less useful than winter tires.
All-season tires lack several attributes which make them less useful than winter tires. They generally are not made of cold weather rubber compounds, they lack channeling tread patterns and come with less tread sipes than winter tires.
Winter tires on the other hand have been specifically designed to be used in the cold season. A common misconception is that you don't need a winter tire if there is no actual snow. That is not true. Winter tires have been created to perfectly suit the cold temperatures in winter, even if these temperatures aren't associated with snow. According to existing research, as soon as the temperature drops below 5 degrees Celsius (41 degrees Fahrenheit), a winter tire performs much better than an all-season one. The lower the temperature, the better the winter tire performs. The exact opposite happens with all-season tires.
On the other hand, once the summer approaches, winter tires should be removed. The same temperature factor will make winter tires wear off quicker because of the softer material they are made of. Probably a driver using winter tires during summer won't feel much of a difference right then, but as soon as the next snow comes, the effectiveness of the winter tire will be severely decreased.