well - to me, "heterophony" just sounds like a pre-cursor/ancestor/poor relative to "harmony" - though I have rarely heard it said in harmony that EVERY voice should be on adifferent note.

I don't normally sing harmony - for a number of reasons - including that I tend to lose the harmony and slide into the melody; but there are a few tunes where either the melody as I sing it harmonizes with someone elses version or by experimentation there are phrases that I sing outside the mainline melody because someone has like it sung that way agains the melody. I guess that would be 'heterophony'