If your pond uses sand filters, it'll eventually get back into the pond, where it'll get drawn into the bottom drain and work its way to a knife valve if you use them. The problem is that some sand ends up sitting on the slotted O-ring which serves as the seal when the knife valve is closed, pushing that sand into the slot where it's stuck forever. This causes the knife to no longer close completely and it'll leak a little forever after. This may or may not be an issue, and though I knew they weren't perfect, they were cheap and they wouldn't get used much, so figured they'd be fine. Didn't think about sand jamming them open though...
I finally bit the bullet and ordered true union ball valves, which are much easier to service, way more expensive, and more reliable, I assume, if they can handle the sand.