Yes, and all four.
I can't speak for a newer Focus w/ all seasons, but my 2004 SVT with summer tires was undriveable with any amount of the white stuff on the roads.
Winter tires have taller, more flexible tread blocks (provide more traction than summer or AS tires can on low traction surfaces: slush, snow and ice), softer compound (provides better grip in low temperatures) and a more flexible sidewall. All of these things make them a better option for the winter season.
Winter tires offer better braking and more traction. How is that a bad thing if you can afford them?