When my daughter was a baby the only way to get her to sleep was to put one foot on her bouncing cradle and play my guitar and sing while rocking her with my foot. I think it depends on the song. Some songs have an emotional impact which is massively increased by singing it unaccompanied, some songs can have a whole new side brought out by a good accompaniment.

When it comes down to it I believe that as a singer of traditional songs it is good to make them accessible to general audiences which can involve musical arrangements, whereas with a different audience which is knowledgeable and interested an unaccompanied version can press very different buttons. All music to me is about communication and as long as you stay true to the intention of the song you can do whatever you like pretty much with it.