Bob Colman pretty much sums it up for me, and I liked Bob Knight's example of Gaelic psalm singing - some of the most incredible sounds I have ever heard come from the human voice alone. To quote Busoni on the potential the human voice has: 'We have divided the octave into twelve equidistant degrees, because we had to manage somehow, and have constructed our instruments in such a way that we can never get in above or below or between them...Yet Nature created an infinite gradation - infinite! Who still knows it nowadays?'
The voice is capable of doing absolutely anything with timbre, scales and rhythm, to which an accompanying instrument cannot always be sympathetic. (One of my absolute bugbears is instrumentalists trying desperately to accompany a song which is simply not meant for accompaniment - e.g. a sean-nos song or ballad that follows no set rhythm - as if acappella singing has somehow deemed to be lacking in some way!)
For me, unaccompanied singing, listening to it or doing it, has a power that nothing else can equal. This point has been made earlier, but I have often noticed in slightly noisy pub sessions that an acappella singer can totally silence a room while those singing with instruments often get talked over. Now of course this is very unfair on the instrumentalist, but I think it highlights the innate power that the naked voice can have. It is certainly one of the main draws for me that Whittlebury has over many other festivals.