"in practical terms though, do these great thinkers on the subject have a message for modern practitioners of the craft?"
Anybody who gets involved in folk song does so at his/her own level of interest; if they are lucky, that level never ceases to expand and develop
I started as a singer, fell into bad company and was encouraged to lift the corner and look underneath the songs - from them on, you never really stop
Not fo everybody, I suppose, but it suits me
Working in Ireland added enormously to the pile of work I wan to do before....
I started a local history course a month ago and find myself stumbling across facts on the locality that are covered by some of the songs we have recorded, the local murder of a landlord mentioned by a singer we recorded who had "forgotten it"
The same singer gave us a song about an incident that happened during the 'Tan Times' which has never been documented and has faded from local memory, but old Mikey Kelleher described as having happened
When we played it to someone from the same area he said, "That's my father he's singing about"

As a singer, the background information adds to your understanding of the songs, vise versa, if you are interested in social history
My concern with research is that far too often the humanity of the subject gets lost in the paperwork
These songs are extensions of life in the past - if you lose sight of that, you lose the point of their existence

I remember the story of two Liverpool kids coming out of a class being held by a very popular teacher, one said to his mate, "Yis gorra watch that bugger, if you let your guard down for a minute you find yourself lernin' summat"
Songs are a bit like that
Jim