Ripov, your statements about classical performance confirm my comments.

"Yes, concert performances of beethoven will be very similar to each other, because the performers all play the same version, and indeed only that one version exists."

One version of the 9th is all we have (but it's certainly enough). Conversely, there is no 'right' version of 'Pretty Saro', there are multiple versions, reflecting the processes that (awkward though the phrase is) we call the folk process. Different versions of Vivaldi's work will differ, but not to the degree that variants of Child #84 differ. There is still Vivaldi's score to refer to; a would-be ballad singer in 2011 has hundreds of recordings, transcriptions and texts to refer to (or he or she can make up his own version).

"The same applies at times to the "folk" scene today."

First of all, today's revivalist folk scene is not the same tradition as the ones documented by Sharp, Lomax and others; secondly, there are plenty of different versions of any given song, each 'right' in its own way, but no single 'correct' version, so I'm not sure the same does apply.