I just bought an extra set of wheels for my 2006 GSXR 600 and haven't used them yet, so I can't really comment on how often you might need them. I bought them because going to these trackdays take a lot of time and preparation (for me anyways), truck gas, packing, maintaining things, etc. and I wanted to be able to ride when I got there, regardless of whether or not it was raining or whatever. I didn't want to have to pay or have the inconvenience of mounting and dismounting tires in those cases, granted you could do eight tire changes and still be under the $350.
I also figured that I could easily sell the wheels if I decided I didn't need them any longer and I wouldn't take too big of a hit, if any unless I kept them for a long time and the bike model went outdated. But in that case, I figured that if I kept the stuff long enough that it went outdated, I would have gotten my use out of them.
That was my logic anyways. It depends on how much the $350 is worth to ya.
-Chris