Thats all very well but at the very least they therefore need to train ALL of their staff that rejection of a ticket by the gates is no reflection on the validity or otherwise of said ticket.
Indeed. What it triggers is a need for the member of staff to properly examine the ticket, find out what the passenger has done and see if it's valid or not.
To use a particularly complex example - take Milton Keynes Central to Bristol Temple Meads, route +Any Permitted, which is valid via London or Birmingham, and let's say for the sake of argument permits Break of Journey on the return half, which it almost certainly does. So on my return, if I travelled via London I am allowed to end short at Bletchley, but if I travelled via Birmingham that would net me a PF. Other than highlighting a need for a "Milton Keynes Stations" group for long-distance tickets (as for anything more distant than London the fares are all the same for Wolverton, MKC and Bletchley anyway), this shows a scenario that it is actually totally impossible to program a whole-station barrier line to handle.
FWIW, the last time I did actually do something similar (the opposite way round - the ticket was from Bletchley and I travelled via Brum and ended short at MKC) the barrier staff asked a couple of questions to ensure I had arrived via Brum before letting me through.
These scenarios are not rare, they occur in many cases where there is a line running north to south but the destination is nearly due east or west. Aughton Park/Ormskirk/Town Green to/from Manchester is another example of this kind of thing, as would be somewhere on the middle of the Cumbrian Coast to/from somewhere in the middle of that stretch of the WCML.