It sounds as if you have had a bad personal experience with these valves Callum?
Not these specific valves Paul, just valves in general. If it isn't absolutely securely closed then should it open while the engine is running then it will drain the tank fairly quickly which isn't a very appealing prospect in several ways. I also have seen a lot of rather more expensive small valves that after a while are not as good at keeping things in as they are supposed to be.
I have a vacuum oil extractor, it is very easy to remove the oil from the tank with it without disturbing the hoses so the drain valve to me is a solution looking for a problem rather than the other way round.
