"learning a song or two is hardly "an exhibition of skills." Children do it all the time."

So explain why adults forget songs. It isn't an easy skill for everyone, especially when you are put in the position of "leader". As you noted "the chances are that the strong, more experience singers probably won't be in evidence" - well, maybe that is the lesser of two evils. I would sooner see someone walk off because their ego and skill level is above a community sing than have someone who is uncomfortable with singing be made to feel foolish for their lack of understanding.

Don, the situation you described at Northwest Folklife is more of a performance since you noted that the staunch singer was facing the audience. It is one thing if you are singing for an an audience that is expecting some sort of "performance", but the sings that I have been used to are more informal and the participation is the draw, not the outcome of the music.

They have been using books for Sacred Harp singing and there doesn't seem to be an issue there.