Embarrassing to admit, but I actually had a 12exp. Hassy back fall off a 500C... I wasn't much given to changing backs, though I had a couple, and usually the back stayed put and I changed film by taking out the carriage. If you work alone, do a lot of shots in a session, then you'd need a hell of a lot of pre-loaded backs to get any advantage. Or rent an assistant, which I'd have hated to have hanging around at those creative moments. Almost as bad as a loose client!