All 8 OW carriers can be e-ticketed. But there are dozens of airports that are not e-ticket capable (including some in the US!). This forces the entire ticket to be paper. Finally, sometimes codeshares affect e-ticketability. Last year I could get e-ticket on AY but not on BA codeshare on AY (apparently AY's computers couldn't access BA's system for e-ticket checkin). These glitches are slowly being solved and e-tickets are 99% available (and work very well -- just be sure to have your ticket number written down, I've had to produce that a dozen times now, and would have been denied boarding if I had not known my ticket number).