My winter wheels and tyres will be going on quite soon. Not only are they better when it's really cold, I've found them very sure footed on damp, slimy, leaf mulch covered roads. There will be people that will say "I've been driving for 40 years and always managed without them" but we also managed without mobile phones and the internet. Research and Development has shown that these tyres do work, so why not use them?
If you already have a spare set on wheels, it's a no brainer really. Not only will the winter tyres perform better, you can only wear out one set at a time, so in the long term, they won't cost you any money. In fact, winter tyres wear better than summer tyres when it's cold (and vice versa) so you'll actually save on tyre wear.
Mine will be on their 4th and probably final winter this time, so replacing them is not exactly a common occurrence.