Originally Posted by jonwvara

Agreed, it should never be used. But the law of averages says that it will eventually get used by mistake. You know, you're thinking about something else, forget what gear you're in, or just accidentally push the lever the wrong way. This is the essence of Murphy's Law, which is often looked on as a sort of joke, but is in fact a serious engineering principle: if something can go wrong, it will go wrong. Systems should be designed (to the extent that it's possible or convenient) so that things CAN'T go wrong.
Infallible riders don't need to think about that kind of thing, since it by definition cannot happen to them. I'm fallible, myself, so I do think about it.