You know, I've been wondering the same things too. I do step on a 10" step, it's small, but I'm budgeting for a nice big step in a few months, and I'm wondering should I buy the extra risers and keep stepping at 10"? I've never done step on anything lower, so I'm not sure how it would feel. I also wonder how long my joints can take that kind of stress. I don't want to lose intensity, either. I suppose you could compensate by making more of the moves lifted and really putting in some height on the moves.
Ok - let's hope we get some good input from the educated crowd on this one.