I'm interested in some of the premium themes, and I appreciate that WordPress has a "Preview" feature.
However, this isn't enough to convince me the premium themes are worth it. I need to try it out with my entire blog and see how I like it, after a few hours.
I'd suggest that WordPress allow users to apply for a 24-hour free preview for premium themes. You could allow users to request it automatically, and you still could control abuse.
I think you'll also see a lot more premium themes.