Craig - "what Hal said"!!
I use my accusump for preoiling for cold starts, and once the engine is started I flip off the switch. Thus no accumulation of "excess"
oil in the sump when the engine stops.
As Hal says - the elec valve acts as a one-way valve when it is not energised.
I also turn it on when I go to road-course track days. I know that under extreme braking, I can uncover the
oil pick-up in the sump. (and hence loose
oil pressure). So the Accusump saves the engine there.
If you had a manual valve, you would have to remember to turn it "off" before you stopped the engine, else, as you say, all that stored oil would drain back into the sump.
All this, of course, assumes that things work the same way down under as they do here. Doesn't the water swirl around the opposite way when you drain the bath tub or something? Could be your oil might behave differently as well. LOL