Nice to see this old thread rear its head again, although I guess not much has changed in the last two years. The last time I did a floor spot I intended to sing the first verse of the song a cappella and then bring the guitar in, but after the first verse I realised that I enjoyed singing the song unacompanied more than with the guitar so continued for the whole song a cappella.
Why not ask "why do we SING ?". Why not just narrate songs. I've even heard talking blues done without guitar acc. Unaccompanied singing wasn't invented in 'folk' clubs, neither is it restricted to our style of western music. There's something magical about marrying a tune to a set of words and letting the voice (the most complete musical instrument) do the work. I've got nothing against using further instrumentaion when singing as long as it something meaningful to the song.