or by hundreds of master song-makers who were either plagiarising one another or else coming up with remarkably similar ideas by pure coincidence?

In the world of traditional craft such concepts of plagiarism have little meaning. To what extent, for example, could a master cooper of the village of Brancepeth in Country Durham, circa 1830, be said to be plagiarising the work of a master cooper in the village of Reepham in Norfolk in the same year, or thereabouts? Both were time-served tradesmen and both are producing faultless barrels in a centuries old tradition of barrel making which is perhaps all but lost to us now. Ploughmen likewise; etc.

I have a notion that the narrative morphology of folk song & folk tale is hard-wired into the human brain as an essential aspect of our preparedness for the structures of language itself. Levi-Strauss extended this to apply to everything from basic syntax through to the classical sonata form. Whilst this doesn't account for specific instances & analogues, it does account for the necessity that drives such a proliferation.   

and maybe we might want to play music and talk about it - though not necessarily simultaneously.

My thing is to play morris tunes on my old Overton low-D tabor pipe in my left hand whilst tapping the rhythm out on the computer keyboard with my right. Thus my particular brand of rhetorical bullshit is born directly of English Traditional Music. In that bit above there, for example, I was playing the version of Country Gardens learnt many years ago from a Folktrax cassette of Kenworthy Schofield; and now, via a cunning segue, I'm playing The Cuckoos Nest as taught to me by Raymond Greenoaken back in 1982, all the while tapping away with my right hand, never dropping a beat...