I love harmonies, and I suspect that even when a song is sung in unison our ears and brains pick up the harmonies surrounding each single note.

I grew up Amish. Congregatinal singing in the Amish church is in unison and yet one of my fondest memories is of listening to the rise and fall of their songs; the tones ranged from bass voices up to the silver sounds of female voices, making a dense river that flowed over rocks and around bends and down the occasional waterfalls, rough here and smooth there...