it is an affectation of the uk folk revival because everywhere else there was no need for a folk revival,
in ireland for example singers and musicians are judged on their performance, not oh we must treat this singer by a different standard because they are a source singer, it is uk folk revival bullshit.
i judge singers on their singing abilty their abilty to put over a song regardless of their label or category, jeannie robertson was a source singer, isobel suitherland a revival singer so what, they were both fine singers
tom brown was a supposedly a source singer but imo only average in tradtional music everywhere else other than the precious uk folk revival, music and song is judged on abilty, it is a peculiarity of the uk folk revival, and particularly the english part because the songs had to be revived.
in jazz people are judged on ther abilty not how they learned their material. if someone is singing any song we listen to their voice which we may or may not like then we listen to how they perform the song.
to treat source singers differently is disrepectful, it is akin to apolgising for them because they are a source singer, jeanie had nothing to be ashamed of she was far better singer of tradtinal songs the same goes for phil tanner, ewan was good but not in their league