I reckon some people tend to not realise or accept that the voice is, or can be, an instrument in itself as used brilliantly by such artists as Anne Briggs, Martin Carthy, Peter Bellamy, Ewann MaColl and loads more. When joined in harmony with others as with the Young Tradition, Wilson Family, Watersons, Firm Friends etc then the sound produced can be magical. Of course some songs sound better accompanied and some people will never take to unnacompanied singing, its a matter of taste really. I prefer a club or festival that has a fine mix of accompanied and unnacompanied songs. As for history, i'm sure unnacompanied singing is as old as the voice itself.
Who's ever heard a thrush or linnet sing accompanied then??