That's complete rubbish, Dick, not everybody can sing well. I'm just lucky to have two good singers in my band to cover for me. And nerves can get anybody's pitch, even very good singers - which is why I like to build an occasional quiet chord into a mostly unaccompanied intro, just to make sure.

And when arranging things it is useful to check with an instrument you pitch at the end of an unaccompanied song in case it has shifted - in which case you have a clue in which direction to change the pitch of the arrangement.