I go along with you to a point Steve T, but claims of 90%-odd percent broadside are both unfounded and illogical.
We have found that wherever there has been a rich recognised-folk tradition this has been accompanied by an equally rich locally composed anonymous tradition of songmaking, distinguishable only by the fact that collectors either missed it or rejected it because it didn't fir the perceived folk repertoire.
Once a literally composed piece arrives in the tradition needs to be examined in the context of the communities concerned and their attitude and access to literacy, which is quite complicated
We have encountered both a mistrust of and a reverence towards printed pieces - quite often they remain unchanged and do't pass into versions.
One of the popular examples around here is Byron's 'Dark Loch Na Gar' - widely sung, but totally as written.
Similarly R L Stevenson's 'Heather Ale' as sung among Irish Travellers.
Unless these songs are examined in the context of the communities they    served and the people who sang them, any attempt to link them with printed versions is rally not worth the paper it is printed on.
There are far too many gaps in our knowledge to make definitive statements.
Jim Carroll