I agree that a sing in which great harmony happens is one of lifes glorious moments. However, another aspect of harmony for a songwriter is that it defines the emotional intent of a song. Just the basic difference between minor and major can darken or lighten a passage.
I tend to sing a melodic sort of harmony line rather than block chords, so that the part shifts through interesting clashes or unexpected chords until it arrives at the expected chord.
Songs like "Anderson's Coast" have recognisable harmonies implied in them, and I tend to write choruses in this way because I've had some great times in singing sessions and want to hear my own songs given that treatment. For those of you who have heard the flood sequence in "Yarri of Wiradjuri", however, I've used a fairly strange sort of "Beyond modal"line that I deliberately intended to create a mood of dark menace.
While it's in shanty form, I'm not sure it's something that folk's will sing in the pub. Good 'ealth! Jack Halyard