It's a term that means a singer of traditional songs who learned them in a traditional manner, from whom others learn songs. Pretty accurate, unless you want to discuss 'traditional'. Value judgements are always up to the individuals involved in a discussion, but the term is one that most will understand.

As far as wishing to argue forever, that's your shtick, Roger.

As to the original question about something being wrong with the term, it gets into the whole traditional/what is folk thing. If, a few generations from now, someone is deemed a 'source singer' because they have a repertoire of songs they learned from their mom, who got it from granny, who got it from etc, and the songs go, "and if you give me weed, whites and wine," and "if I had a hammer," and "we all live in a yellow..."

See, Norma Waterson is a source singer for me, but I'm pretty sure she got involved in a/the revival. (Could stand to be corrected here.)

The people we call 'source singers' are really passers-on of songs. They may be a 'source' for all of us outside their tradition, but inside their tradition, they just learn the songs from someone a generation or two before - their source - and pass the songs on to the next 'source', until someone from outside can discover them and call them a 'source singer'.