The thing is that there is no real huge difference between Spirit and the legacies once fees are added on. Really, do you want to bring your own water bottle with you? Do you want to chance ending up in a 28 inch middle seat between Fat Albert and Buppa? How about if you did not/could not print your BP (AFAIK Spirit does not have a mobile BP)? What happens when your flight is canceled?

I think the Ultra low LCCs take advantage of the stupid factor.