I believe from an artistic standpoint, the variation in delivery of a song or an instrumental rendition from one performance to another is the most fascinating part. I don't want to hear [artist x] perform a song that is a carbon copy of the recorded version, I am thrilled by the new interpretation and subtleties of delivery that live performance brings and might have been opportunistically captured by an enthusiast for posterity. No, I do not necessarily want to hear Sandy Denny sing *every other* performance of "Matty Groves" any more than she reportedly wished to keep on singing it, but among the live performances out there there are some I am very glad to hear.
What will the musical appreciators of 100 years time wish to hear, from a time before they were born? Nobody knows, but I do think we should not be throwing out such recordings as exist, whatever the circumstances of their collection. Do we throw away Van Gogh's or Picasso's early work because they later thought it was not very good or representative of their later efforts? No we do not, we cherish them as a window into the artist's development and creative process. Do we care that they might have been stolen from the artist's wastepaper basket, if that is the case?
If I had an unauthorised recording of Robert Johnson playing in a club in 1938 to complement his official recordings, would I be destroying it now on moral/ethical grounds? I think not, and collectors, other musicians and the general public might be just a little interested to know... who is to say that in 100 years, my live recordings of Dave Swarbrick, Vin Garbutt and Jez Lowe (to name a few) might not also be sought after - we just don't know. (Of course if there are many other similar performances on record then the vlue is diluted somewhat).
Just my 2 cents of course.
Regards - Tony