I'm also an enthusiastic chimer-in. Although there are definitely access reasons for people to talk strictly one-at-a-time: recognize who is speaking, for speech readers/interpreters/captioners to keep up, and for people with cognitive issues (me!) who cannot multi-task at all.

When people are making "yes!" "I agree" or "nope! not for me" sort of noises/non-verbals, that's fine. But if two people are actually talking at the same time, I can understand neither of them.