I'm of the two sets of wheels crowd. Last year I swapped tires back and forth on one set of wheels and that got old in a hurry. I purchased a nice set of Mavic CrossMax wheels that I put my MTB tires on--definitely and upgrade from the stock wheels. I then put my XT cassette on the Mavic rear wheel. On the stock rear wheel, I put a 105 road cassette and some 1.5 inch slicks. Both cassettes are Shimano 9 speed so they work great with the indexed shifting. I run a tight set of gears for the road/commuting 12/24 and a wider set on the MTB wheels, something like 11/35.

Now when I get home from a commute, it only takes me about 2-3 minutes to switch wheels and head out on the trail! Plus, I feel like I have two really high quality bikes that both fit me perfectly, for much lower cost and I don't have to do maintenance on two bikes.

I also don't see chainwear as an issue. The only problem you will have is if your current cassette is in bad shape--and then you need to replace it anyway.