PaperTiger:
Paper tickets on US domestic have been on the way out for several years. In fact all US airlines now charge you for issuing them in the first place.
Not entirely true; they still issue paper tickets at no charge for situations where e-tickets are not an option - for instance, an infant not occupying a seat.
If you lose/misplace/forget a (paper) ticket or e-ticket ref. number, you will be required to purchase a replacement.
Certainly not the case for e-ticket reference number. I frequently don't have a printed e-ticket, I just give my name and ID and they find me and check me in. I can't imagine any circumstance you would have to *purchase* a replacement e-ticket!
R1