Depends a lot on how often you change out tires. The wear thing is most true in the cases where you ride one way for months then change. That works out nice for once as that is the case where spare rims save you the least bother.

There is however a second reason to get at least a second rear rim and cassette. The gearing that makes sense for offroad use on dirt is not the same as what makes sense for onroad with slicks. When I replaced the tires on my Mtn. bike with slicks I found that on any straight level road I was almost always ran out of gears at the high end.