People singing on top of each other is just about as rude as people talking through each other.
Another more technical reason is that vocals happen roughly in the 2 to 4 KHz area. Engineers take lot’s of care to clean up that frequency band to bring out the vocals and not have them “drown” in the mix. Mixing in another set of vocals therefor usually almost doubles the energy in that frequency range, which throws off the total balance of each track individually and of the combined sound.
So, listen to the old hacks around here … don’t mix vocals on vocals!
Greetinx,
C.