Let's assume that the usual (maybe objectionable) use of "source singer" is to indicate that the singer is to be taken as a stable, authoritative provider of the song "as it should be", because (s)he is the modern flowering of the tradition, whereas the rest of us are passive collectors, echoes of the tradition, as it were.

This suggests that the life of the song in tradition has come to an end with the "source singer".   It may suggest that any further developments of the song are illegitimate, more modern corruptions of the traditional song. At best it means "This is a singer, rooted in the tradition, from whom I got the song in question," or "from whom one may get genuine traditional songs." That's at best.

It also might suggest that the version sung by the "source singer" is either the best or the only legitimate version, or perhaps the finest flowering of the song's life.

Alternatively, the term source singer might, to some minds, seem to refer to the originator of the song.

That singer, no matter how well (s)he does the song, no matter how knowledgeable the singer may be, no matter how well that version may reflect the song's essence in its traditional life, is neither the writer nor the last word on what the song is, has been, nor can (s)he be.

Dave Oesterreich