Put you coat away, Nigel. Recordings are as different from a live performance as movies are from the books they are based upon. When you take away the visual presence of a peformer, the song introductions and the interaction of the audience, there are times when the addition of an instrument or two, or some vocal harmonies can give the song the richness it would otherwise lose. Of course, there are many exceptions. I wouldn't want to listen to a Mississippi John Hurt recording with drums, a bass guitar and the Hurtettes singing backing vocals. But, not everyone is Mississippi John Hurt, Doc Watson, or Doc Boggs, for that matter. And, the last time I checked, this isn't the 1920's in the Appalachian backwoods.
I think it's up to the musician to play the music with the arrangement and the instrumentation and vocal harmonies they feel are right for the song. It's up to the listener to decide if they like it.

As it has always been.

Call me a Folk Impurist

Jerry