Re: Can they do something more useful instead?
"how hard is it to change from summer to winter tyres? Is it basically the same as having four spare wheels but with the other type of tyres?"
Yes, unless you want to pay a garage to change the tyres on your existing wheels.
No, it would certainly not be practical unless you live in the mountains of Scotland or similar.
I think a set of wheels with winter tyres would cost more than my car is worth. Some folks have no idea how much some of us struggle to make ends meet.