It's not mandatory to have winter tires in Alberta, so until it is, it's all seasons for me. It is mandatory in Quebec though, isn't it? I just don't see the need to have a set of winters. Plus, living in an apartment building, I have nowhere to store tires.
Anyway, the city do a great job of clearing the major routes, so the roads here are never that bad that winter tyres are a must, IMO.
The Optimo 4S have a pretty good snow/ice rating for an all season, so I'm happy :-)