As a mechanical engineer, I totally agree with Trev on this one. The oil pumps used in our BSAs are nothing but simple external gear pumps. In it's basic design, an external gear pump is not working well under critical suction conditions. This is a matter of fact and each serious book on work machine or hydraulic engineering will tell you so.
Have a look at this link, esp. at the part of "Installation issues":
http://www.e4training.com/hydraulic_pumps/B1.htmImho, if the pump is not leaking, the shut-off valve is not likely to cause any trouble, because it is easily sucked open by the primed pump. On the other hand, if it is leaking, it will run dry and might not be able to create enough suction to open the ball valve that is cutting off oil supply. The BSA engineers did well know why to put the ball valve on the pressure side of the pump and mounted the oil tank far higher than the pump itself to ensure proper priming.
The ball valve in the sump is a completely different story. It is NOT spring loaded, unlike the anti-wet sumping valves, so the ball will easily be lifted off it's seat, once the sump is running full of oil, so oil is forced to the pump within seconds just by hydrostatic pressure, if the oilway to the pump should be empty for any reason (like after a rebuild, for example)
And have you ever asked yourself why the BSA engineers fancied to put this valve there? Imho it's only task is to keep the oil in the oilway leading to the return side of the pump when the engine is stopped, in order not to have it run dry at startup.
Just my personal opinion, but I consider these valves to be a rather safe way to a complete engine rebuild, sooner or later. And if your oil tank is empty after just a few days without riding, you should do something about your leaking oil pump. It is a matter of curing the illness, not the symptoms.