Yes cold temperatures affect the performance but snow tires are not necessary with cold temperatures. just dont drive as if you are racing in cold weather.
but the question is, what are the odds of having 3 consecutive historically warm winters in a row?
so probably a cold winter is coming next year.
The other advantage of having a second set is (based on my recent experience) if you destroy your tire on a saturday when dealers are closed. you can switch the 2 tires on that axle and continue driving until the dealers open, order the tire and have it ready.
It also pays off when you drive to a ski resort and you end up having a blizzard. I got out of 8 inches of snow with these tires when my friends X6 got stuck. (it was very embarrassing for him)
BMW M3 E92 - Silverstone II
BPM Sport Stage II Tune + Test Pipes, GTS DCT Tune