Well, yeah, most people want two LS-30s so they can play games like Ikari Warriors, Victory Road, etc, since many of the rotary games were two player simultaneous games. I can't think of any single player dual rotary joystick games..
If you wanted to play any of those games solo, sure, all you need is one joystick. You should also pick up some sort of an interface (i.e. Druin's interface). However, if you're not familiar with rotary style games, you might want to fire up Mame and check some of them out and see if setting up a rotary will be worth the effort for you.
I for one can't wait to trick out my 49ways with Floyd's rotary hack!
-88mph