Roxy, the answer to your curiousity is that a lot of people who do this are students, and the rush prices often make it more affordable. It's still certainly not cheap to run with habits like seeing a show twenty or thirty times, but the smaller repeat digits are do-able if it's only costing you $20 or $25 each time. That's not much more than dinner and a movie, really.
DP, while I disagree with you, I can see the "see it once and move on" sentiment, because I think there are a lot of people who feel that seeing something more than once can sort of kill your initial memory of it and burn it out, or that you should try to expand your horizons and see as wide a variety of things as you can rather than seeing the same musical 25 times. For a lot of people, I think it's the idea that you're falling in love with something completely temporary, and that you want to enjoy it while you can. When it's gone, it'll be gone forever. That said, though, I'm gonna
hope you were being facetious with your "forget it" part. That just seems ridiculous to me -- see something and then just put it out of your mind? Isn't that backwards, to expect theater to be forgettable? One would think the point is not for you to just enjoy it while you're there and then be able to erase it from your memory, but for it to stay with you.
A work of art is an invitation to love.