The main case for break of journey is popping to the shops between trains, or stopping or starting short for non-fare-avoiding reasons. I've never been refused that even on an Advance, but it does seem enforcement is getting stricter over time and there has been the odd anecdotal evidence of it being prevented even though industry guidelines are not to prevent it. It's that that I feel really needs to be protected.  Though the single-fare model does reduce the need for that. I often buy an Off Peak Return northbound from Bletchley but start at MKC so I can go back to Bletchley if I want. With a single-fares model there would be no need to do that.