I have always had two sets of tires, and would even if I didn't ski as often as I do. My thinking is that a good set of snow tires is about as much as most peoples' $500 deductible, and they last several years.
If you've got the cash and some storage space, buying a cheap set of wheels to go with your snows is the way to go. That way you don't have to pay to have them mounted/unmounted twice a year, and you can put them on whenever you want.
I've had Blizzaks, X-Ices and General Altimax Arctic. I would recommend the latter, because they were just as good as the other two, and cost me around $400 a set, shipped.
Being able to be sure that I will get to work and that I can stop when needed makes it worth it.