Even without any frost, if the road temperature frequently drops below 7 degrees Celsius, than winter tyres are desirable.
I am all for concentrating on the driver, but a cooperative car can make a noticeable difference. And among what makes a car cooperative, tyres are by far the most important part, being the only part that actually touches the road.
I see lots of old tyres, or even new tyres that come from questionable manufacturers, that take significantly longer to stop. If the need to stop suddenly arises (which it might, even for the best advanced driver) that difference could mean everything.
Here's an example of what the difference between tyres from a known and unknown brand look like in similar conditions at 60mph. There's 25 feet between the two, in the dry!
The differences between winter and summer tyres are even larger, and the difference between new and aged (3-4 years) tyres is slightly smaller, but nevertheless significant. The same can be said for suspension parts, too.