Some interesting reading:
http://www.autotirecarcare.com/guide/speed.html
You can run a winter tire all year, but why would you? I'm running Michelin Arctic Alpins this time of year, the PT rides and handles like it's on marshmallows. The tires are soft , they wear quickly, they corner like they're about to roll off the rim. But they're great on snow and ice, that's why they're on the car. They come off the first weekend in April.