"Have there ever been any (genuinely) commercially successful purely unaccompanied singers? "
"The Flying Pickets"

If I'm at a 'Filk' convention I generally sing unaccompanied. Mainly because I lack the playing ability. But as German says it does mean that people often listen more closely as the message is carried purely by the words, and no accentuated (or hidden) by fancy guitar work

Nigel