I can't agree more that "knowing what the song is about" is paramount in importance. I know people who couldn't care less about the lyrics or even the substance of the song. They have a hard enough time just plinking along with it on whatever musical contraption they have. Often the only way they are even aware that the end of the song has come is because everyone has stopped singing!

That "the words are just the starting point" explains the folk tradition in an economy of words. I think the invention of new verses or the partial, or even complete substitution of half-remembered lyrics is the basis of what some of us still refer to as "traditional folksongs." The basic sense of the song was maintained, (usually,) even when someone changed the lyrics a bit - whether by intent or by accident of memory.

Don points out an instance of the very thing I suggested. If there are hardliners among a session or song circle who cannot abide crutches, then either the hardliners or the cripples may need to find or to create a different venue.