I think this thing about memorising trad songs is something about caring about your music. For the last year or so I've been supplementing my salary by playing in bars around Recife, doing Beatles, Simon and Garfunkle James Taylor (someone else normally plays that with me as i am yer basic trhree chord kid) et al. Real MOR stuff that I just don't care about. And i use a song book...i haven't memorised nary a one, why? Because I don't really care about them. But a folk song be it trad or contemporary, I wouldn't dream of getting up and singing it from a piece of paper. If I can't remember it, then I don't sing it. Simple as that. But that's why I used to love folk clubs because you couod stumble your way through new material in a singaround and refine the song from there on.