A lot if it is a poor design, but then you may not care if it leaks a lot.
If you want to close off flow of mud, then the valve can be fairly sloppy. If you want to close off flow lava, then you want to make sure you are not using soft seals. If you want to close off flow of liquid helium, then you want to make sure you take CT effects of the valve seal interface.
.....or generally: The leakage rate and design must be good enough to do the job the valve is intended to do over the planned lifetime, and the constructional cost of the valve design cheap enough to be attractive on the market against competing constructions.
