I saw a product review that was complaining about the built-in microphone of the camera, and in one paragraph stated it was 'all but useless'. I have seen 'all but [negative word]' in different places before to describe something as bad, but shouldn't it mean that it is everything (including good things) except the bad thing; i.e it is not useless as it is all of the good things but not the useless one?
Could someone please explain the correct usage, and why people do/don't use it?