Memory certainly isn't perfect and can be as troublesome as the written word, but for different reasons. Just remember how many fragments were collected by folk song collectors. Some were made up into fuller songs by borrowing from elsewhere (Brigg Fair being an example); in other cases there was no elsewhere so we remain with fragments (Orange Tree being an example. Never heard of it? The entire words we have are "'Twas in the month of May / When flowers they were gay / When the orange trees were all in bloom / When the orange, orange, orange trees were all in bloom").

*Good* singers will not simplify. And good singers will treat the tradition as a resource to be modified, thus moving it well beyond the written word (for example, Martin Carthy).