Interesting that most (not all) of the comments above - and I do agree with many of them - seem to be from the viewpoint of the solo performer. It may seem a small point but the dynamics of the thing change when there are two performers, or three or more, etc.

As a singer of 1920s and 1930s trivia - not a profound thought in sight - I like to balance the words with a little bit of instrumental playing in the middle of the piece. (And I should stress that I sing the words, rubbish though they may seem to be, as clearly and with as much meaning as possible). Furthermore, when I'm playing with my old buddy Chris Wolferstan - one of Sussex's finest guitarists - we play very much with a "jazz-ish" feel. By "jazz-ish" I mean that we play, on the night, very much as we feel it, with as much improvisation as is necessary and no more. If the song is going well and we sense that the audience is in the groove with us, then we might throw in an extra instrumental or vocal chorus on the fly (pun intended).

The emphasis, therefore, is not just on the song, or on the instrumental part of the song - but on the audiences' enjoyment - making sure that the people in front of us are entertained and having a good time. We're not out there to change the world - other people can do that - we're out there (hopefully) to bring a little fun.

I should stress that this is from the perspective of someone who doesn't sing folk songs. Traditional tunes played by the dozen - yes - but not traditional songs.