I can see your point Jim 'diddling' or 'mouth music' is using the voice as a substitute musical instrument. But as you say it turns to the philosophical and semantics and subsets when you have an ordinary singer. As far as I'm concerned my late father was a singer not a musician, and he used musicians to back him up. As a whole band they were musicians. But within the subset my old man was a singer, not himself a musician    Whereas my brother who was originally my father's backing musician and harmoniser -is a musician who now sings to his own backing! This one just goes round! Everybody is right though! My head hurts!