Songs should be judged on their quality as a song.
Here is another viewpoint, all "traditional" songs were originally compositions, but through the mists of time we have lost their authorship, if that is the case they should be judged in exactly the same way as modern composed contemporary folk songs.
I wonder do folk singers dismiss a song written by a popular singer such as Harry Chapin?, The Shortest story, because he is not from The Folk Revival.
http://www.youtube.com/watch?v=ix-Yj_Md2no