I, too, wonder why Amado water keys are useless. I've never had one on one of my main instruments, but I've seen other people use them and they did seem to work.
The thing I don't like about the Amado Water Key is the fact that if you let it dry out (by neglecting to play the instrument for a few months or years) it gets stuck. It is possible to free it up with the right tools, but it's a pain to have to do so. I'll take an old-fashioned water key with a rubber band around it.
