In practice, there are lots of cases where stopping/starting short is either common or being caught would be very difficult, particularly if people stop short at an interchange station where they switch immediately onto another ticket.
My dad has been starting/stopping short on Advances for years, as he lives between two stations, one with Advance fares and the other slightly closer with slightly fewer services stopping and no Advances. Clearly, local guards aren't bothered about it as he does this at least once a month and, particularly where starting short, they must be aware of people doing it.
But I'd never recommend it to someone unless I was sure that they were fully aware of the possible consequences.