Frankly, John, when, on weekends, I go through my repertoire from Hymns Ancient and Modern and see 3 lines other than what I'm playing, I think it's beyond my musical abilities - certainly on the tenor-recorder! and on keyboards I use both hands for just the top-line.
But, on top of that, if you'll pardon the pun, I really do like hearing just top-lines well played or sung, and that's one reason why folk is my favourite genre.
Also, I think that a lot of folk in pubs (if not the stage) is just so nowadays - at sessions most are playing just the tune, yes?..and, at the singarounds I attend, more than half are indeed singing unaccompanied.
And I shall check that shortly, Marje...I'm wondering how many part harmony..?...all singing just the tune in close harmony, or something more sophisticated..?..art-songs from folk-songs, which apparently (I watched an early music series on the BBC a couple of years ago) also goes back centuries..?