The point about capo one (or four) is that it takes you into keys that many instrumentalists can't play in. By now I have got used to this and have a few instruments in weird keys so I can play along with folks like The Lady Who Does "Ride On" In C Sharp Minor, but most whistle and harmonica players aren't that prepared.

Most singers can move up or down a semitone to a key other people have a chance with. It's just plain rude to cut people out with the funny-fret trick.