I think it's a very useful term which could be applied, for example, to "You'll Never Walk Alone" which is a composed song that has entered the vernacular of Liverpool (I remember someone in another thread quoting AL Lloyd as saying this song was "folk by usage but not folk by form").
And I have heard the term used in a folk club. A gentleman with a thick Wiltshire accent came up to me and said: "I like your songs. You write them in the funicular." It did take a moment or two work out what he meant but I'd like to think he's right.