I live in Minnesota. I've seen lots of people recommend changing to winter tires in the winter. However, what doesn't make sense to me is that the roads are plowed very well here. 90% of my driving in the winter is either on dry or liquid-wet pavement; it is rare when I drive in the snow or ice. So I should get winter tires for conditions that are only in effect 10% of the time? Why not then keep the winter tires on year-round?
As I said before, it's a trade-off: All weather tires vs winter tires
It depends: how bad is that 10%? Is it the difference between well-plowed and deep snow with a slush garnish? Are you trying to improve the handling or are you worried that you'll:
I live in Rhode Island with all-season tires and our snows are generally minimal or manageable, well-plowed (especially after I've finished shoveling my driveway) and I avoid driving when I'm worried about other drivers.
Even so, I still wish I had dedicated snow tires for exactly that 10% when everything goes wrong and all the margins of error become much tighter.
Winter tires will give poor performance on dry hot roads when considering handling, top speed and road noise. They're wasted in warm weather unless you decide to try rallying or rallycross.
EDIT: Holy cow. The winter of 2014-2015 has been crazy even in Rhode Island! This year, I bought nice winter performance tires and I'm so much happier. The performance envelope is much wider, giving me a lot more room to avoid the crazies who shouldn't be driving even on a dry summer day, much less in the snow! I'll be getting new summer wheels and tires in the spring and switching back and forth from now on!
For the part of the question of why not just leave winter tires on all year, see Why should winter tires not be used at summer? (essentially, they aren't made to handle summer conditions)
It isn't purely about surface conditions, although that is a big portion of it. The compounds that winter tires are made out of are designed for the lower temperature ranges.
Its the simple jack of all trades master or none statement.
Winter tires excel in cold weather, will fail in summer.
Summer tires excel in summer and will fail in winter.
All-Season tires will work in both, but cannot match the performance of the specific season tire.