When I went to Steve Roud's presentation of his book at Sidmouth - I believe he clearly stated that his primary interest in a song is when and where is was performed in the voice of the singers.
Therefore It didn't matter what it's source was, it was the fact that is was sung over time, and possibly altered, to suit each singer, that most interested him - and that he considered these Folk Songs.
I am probably Paraphrasing very badly......but that was certainly my impression.

Tim Radford