Back in my folk trio days, when we sang unaccompanied we sang in three-part harmony. Two of us, however, were harmony-impaired, so it became crucial to craft harmonies that were (almost) as innately singable as the melody...somewhere between what Wysiwyg calls heterophony and "simultaneous horizontal melodies." That way, the harmony-impaired could learn their parts as melody lines and pretend they were singing lead, which helped them get it right.

We used unison, too, both across the trio and between any two of the three of us. Sometimes we used it as as punctuation on important lyrics or to accent a key melodic bit (like what you'll often find in church music). But more often, we doubled up on notes to insert a stripped-down chord (tonic and dominant only, tonic and subdominant only...that sort of thing) between more complex chords as "ear relief" -- a necessary "rest" for the audience because we tended to use rather bizarre chords/harmonies. (Good, though...)

Why I started this is because once, for an a cappella competition, we commissioned a vocal arrangement (of Eric Bogle's "The Moggie Song") from a professional musician. She chose to write our arrangement in "barbershop," a style about which we knew next to nothing. Her work was spectacular, and in it we never came together on a single note. But gosh, was it hard to learn! The intervals were all dimished fifths and augmented ninths -- a memorization nightmare. Very inorganic, somehow counter-intuitive...not a harmony conceived in nature, meaning you'd never burst spontaneously into one of those parts. But impressive -- heck yeah. And fun to sing, once we'd learned how to do that.

Speaking as the token non-harmonically-impaired member of the aforementioned trio, I think that in the average melody there are places that cry out to be enhanced with rich harmonies -- and other places that are best left more austere through the use of unison or two-note harmony. I think that varying the level of harmonization strengthens an arrangement by adding depth and texture.

That's my story, and I'm stickin' to it!