It all depends how you use your car right?
Like yourself, I never really saw the need, but having had a heart stopping moment in the alps when I got caught in a storm I bought a set for my next winter trip down there. The difference was amazing and I decided that they are worth the cost for my safety. We run them on both cars in our house hold now. Arguably, they're less likely to be needed the further south in the UK you go, but where I am is pretty hilly so they do serve a purpose in insuring you don't get stuck, and more importantly helping you stop in an emergency. People always argue the case for that point, but they forget it also gives you options. Say you have to stop in an emergency, but then the car behind you gets in to a skid, at least with the winter tyres, you habe the option to quickly move your car and get out of there way if possible.
As for cost.
Well, yes, it's a significant initial investment, however, once the tyres are bought, your summer tyres last longer meaning over time the costs even out. I also think the spare wheels save diamond faced wheels, so again, the 21" option wheels continue to look great in summer by not getting destroyed in winter.
It's all personal preference.
