Interesting thread. When I sing with my my two partners we generally avoid unison, but we find that the three of us singing the same note in three octaves can produce an interesting effect if used sparingly.

I read somewhere that wolves avoid unison when howling. I've played with that notion in a dog yard full of huskies. Once you've got most of the yard going (in the case of the dog yard I know, you can have a hundred dogs in on the sing) you can only hear the five or six dogs closest, but they seem to choose different notes. Howling the same note as a nearby dog often causes him to find another note for himself. Try it. It's more fun than rehearsing and you don't have to buy beer.

D