Steve,
On the other hand - some time in the late 60s a song was composed about a 'made match' between two Travellers. 'Made matches', marriages agreed by the parents of the couple and finalised by a go-between 'matchmaker' (usually with the complete agreement of the couple) were common in rural Ireland right up to the 50s and among Travellers possibly into the 70s. It was said to have been made up on the day of the wedding by a group of guests. Nobody knows who the makers were; so far we have come across around eight versions of the song.
I have no doubt that what you say is correct but I would suggest that the vast majority of versions of folk songs are down to the oral tradition (which I am quite sure you are not doubting the existence of).
Work we did in West Clare points to the fact that not only was the oral tradition very strong here, but songs learned from 'ballads' (printed song sheets), underwent significant changes almost immediately after they were taken up.
Exceptions very seldom prove rules.
Jim Carroll