Elton John once released a single of Lucy In The Sky With Diamonds, more or less faithfully replicating the Fabs' original arrangement of the song. When reproached for this by pop commentators, he made the interesting point that a pop song exists primarily as a recorded performance (many more people buy records than buy sheet music), and that the instrumental texture and arrangement was therefore felt to be an integral element of the song. Brian hints at this with his pleasing fantasy of Johnny B Goode with highland pipes. Folk songs, on the other hand, historically existed either as texts or as unrepeated performances, so singers tended to take the bare bones of the text and tune and do it in their own way. But as soon as a folk song is enshrined in a recorded performance, that performance potentially becomes definitive, and that's how everyone feels the song to be. How often do we hear reviewers tutting: "Not bad, but it can't live with Dick Gaughan's version" etc.? Once upon a time the folk said: "How does it go?". Now they're more likely to say: "How do you play it?"
By the by, Brian's Persistence Of Memory lp will set you back £15 quid at Sheffield's Record Collector.