If I may introduce a note of curmudgeonly scepticism (Merry Xmas, Mr Scrooge!) - I hate harmonies! This hideous, self-indulgent droning far too often spoils my enjoyment of a perfectly good song. And certainly on the British folk scene the popularity of many harmony groups, from the Watersons onwards, have contributed, in my opinion, to that most horrible of phenomena the dirge-like chorus/refrain (picture dozens of bearded blokes, tankards on their belts and clogs on their feet, eyes closed and fingers in their ears and all singing through their noses, in a variety of keys, the umptyninth interminable chorus of 'Fathom the Bowl' or 'The Wild Rover' - deadly!!!).
I can't speak for our friends across the Pond but I believe that the English Folk song tradition (and Scottish and Irish as well) is a melodic and narrative one and harmonies have a tendency to smooth out the (often very beautiful) tunes and get in the way of the story. Ban harmonies - that's what I say!