Anyone can be a 'good singer' and so worth recording as a singer per performance. But to be worth recording as a Source Singer should be a function of their proximity to the oral tradition. Did they learn a song from a parent/grandparent who is directly linked to that tradition? If so, the version of the song is worth recording. If in addition they were likely to have been, when they heard it, mature enough to have been able to assume anything of that singer's style they may also be worth recording as a Source Singer in performance.
Many may say that this criterion is too narrow. I make no apologies. We are the generation under whom the continuity of the oral tradition died. Our first - I think our only - responsibility to coming generations is to save every last piece of evidence of what that living tradition was like.