I find this a very interesting thread, and from the varied replies it is obvious that you cannot pigeonhole singers simply as revival or traditional. It's a lot greyer than that. In my collecting activities I have recorded singers who learnt their songs from a variety of sources, including friends, family, 78 rpm records, etc. I show more interest in the songs which have come through an oral tradition simply because, as someone said above, there is an element of 'musical archeology' in this subject. If an illiterate traveller sings to me, say, 'The Cruel Ship's Carpenter', then that gives me a buzz to think that I am hearing something which has been handed down like an heirloom. However, for aught I know, that song might have been learnt from a printed broadside 2 generations previously! So what I am saying? Well, what I think I am saying is that in order to understand the 'folk process' we need to take a broad look at a singer's repertoire, background, motivation and singing context, and not just note the songs.
So where does that leave us? Well, I learnt 'Around her leg she wore a yellow garter' from oral tradition. Does that make me a traditional singer? Perhaps half of one percent of one. I sing songs I recorded from source singers - does that make me a source singer myself? Think about - I am just one link in a chain, as was my source singer. Some of the gypsy singers I recorded learnt some of their songs from George Formby and Jimmie Rogers 78s - does that remove their credentials as 'source singers'?
I think what it comes to it is this - by source singers, we are talking about people who have not learnt their songs from the folk revival or the media. However, there will be numerous exceptions to this.
This is a bit rambling, but I hope it shows that it is dangerous to label singers one way or the other. I have been collecting for 40 year and I am still not sure of the answer.