alt Hacker NewsPlease read the studies in this thread.
Modern high quality all weather tyres are excellent in summer and winter.
Except on actual snow, where they're just ok, because of the hybrid sipe patterns, and ice, where they suck exactly as much as everything else except studded tyres (which suck on tarmac instead).
Even after the streets are plowed there's still a bit of snow on the main roads and a lot on the side roads. Maybe you live in a place with really mild winters, but my car would have drifted into a ditch many times this winter if I didn't swap my tires.