Bob Coltman's post put my position rather well. There are some songs I sing (and have heard sung by others), that become diminished if sung by more than one person at a time. "My old black billy" is one such (although I have enjoyed Mike and Tony harmonising the chorus) and it's very difficult to prevent such songs as "The streets of Forbes" becoming truly awful when accompanied by instruments. But there are also songs that, to me, beg for harmonising by voices only and suffer when instruments are employed; those who've heard Coope, Boyes and Simpson, or the Spooky Men's Chorale will have heard examples. I appreciate the arguments for not singing to the "Last Post" (which I've always understood to be a bugle call rather than a song) or "Taps" (the US equivalent, for which there are words) and echo Joe's comments about not using instrumental accompaniment to Gregorian chant. I've yet to discern any "sacredisation" (or equivalent) of voice-only singing, though.

I'd extend the notion that some vocalisations are best left without unnecessary ornamentation, so to speak. Oz has a tradition of bush poetry and prose recitation, probably inherited from the British Isles; I'm not sure how much of that tradition is shared by US contexts. Many poems that originated as recitations have been ornamented by having tunes applied or written for them, as that seems to satisfy some modern tastes, as well as satisfying the needs of those who wish to perform them but aren't satisfied with the power of the naked words, or perhaps they're not satisfied with their ability to convey that power without a tune.

Lots of songwriters have regarded Henry Lawson's poems as fair game but, to me, most have lost much of their power by being sung rather than recited. Often the tune forces its rhythm, overriding the freer phrasing that recitation allows. In the hands of a sensitive performer this can be overcome (Cathy O'Sullivan's "Deeper Down", Lawson's poem of sinking artesian bores is an excellent example of how a poem can work well with a tune) but all too often the music (for me) distracts from the poem rather than augments its power.

The metaphor of graphic images, alluded to above, is interesting. Some images need full colour and some songs require arrangement of much instrumentation. Some images, such as Arnhem Land rock art and bark paintings require a (comparatively) subdued palette. Traditionally, that was all that was available, although Nourlangie and the Desert Mob show what happens when choice is widened; some instrumental and a capella harmonisations are similar. Some photographic images work best in black and white the way some songs work best as solo voice pieces. And, the way some purely line images (black on white or vice versa) starkly and sublimely convey their power, so can the spoken voice; Emmet's speech before his sentencing, or Lincoln's Gettyburg address are two excellent examples.

But, to each their own.

Cheers, Rowan