"Do you still really believe this stuff?.......
Absolutely Steve - for the reasons I have given - vernacular, familiarity.. etc, also, how the songs sit within the tradition, and the proprietorial attitude adopted by the singers towards their songs.
"Give me an example of a forebitter made up by a sailor!"
I will happily Steve, if you, or anybody can produce anything like convincing proof of the authorship of our traditional songs - broadside printers or otherwise (other than a "gut feeling".)
Do your really believe that a Seven Dials denizen could produce anything as authentic as 'Round Cape Horn' or Harry Cox's 'Van Dieman's Land'?
This area of West Clare has not only been a gold mine for traditional songs, but also it has been an extremely active song-writing area, particularly in the early years of the 20th century. We have folders of locally made traditional songs (or traditional songs in the making) on subjects like the Irish War of Independance, fashions, fishing disasters, sprees, The West Clare Railway (4 songs on that), the singing of the French ship, The Leon XIII, musicians, emigration (dozens).... - all obviously locally produced - and nearly all anonymous - though they must have been made within the lifetimes of the singers we met (and got some of them from). Can you come up with anything approaching that?   
I'll show you mine Steve if you show me yours!
"I would be interested to learn on what evidence people who make this claim base it on."
Would thirty odd years of interviewing source singers do, Ernest?
The results of this can be found in our collection in The British Library, but you might like to read what Walter Pardon and others have to say on the subject in my reply to Mike Yates' 'The Other Songs', making a similar point to your own ('By Any Other Name' - Enthusiasms section, Musical Traditions (can't remember the date).
I really do get a little tired of our traditional singers being regarded as merely sources for songs and not the sentient human beings they most certainly where - If I can tell the difference between 'Knock 'Em In The Old Kent Road' and 'Tiftie's Annie' why on earth shouldn't they be able to????
Jim Carroll