Well, on a trainer you don't have hills to go up, descents to come down, and you can't really sprint. So I tend to use a cassette with closer gearing than I do outside. Outside, for training rides I normally run an 11-28, when racing I'll swap to whatever fits the course (11-28, 12-25, 11-23, whatever) But my trainer rear wheel almost always has a 12-25 cassette on it (unless I swapped it for a race).

And if you haven't figured it out by now, cassettes are pretty interchangeable. Wheels, too.

So a spare wheel/cassette set up for trainer use is a pretty good idea. And FWIW, the cheap 10-speed Shimano Tiagra cassettes shift just about as good as the more expensive Dura Ace ones do. And no one's going to be able to tell which one you have on your bike anyway.