I have to admit I'm not familiar with dual cassette hubs, but I gotta say that switching out a tightly spaced road cluster for a wider range touring cluster is a really easy job that only adds about three minutes to the time it takes to change a wheel.

That said, I'd say having a second set of wheels/tires is a great idea, allowing not only specialized setups but also a nice little backup in case of (god forbid) an accident or a wheel goes in for servicing.