How much is communication of information the main purpose of speech? I can think of other reasons to speak:
1. We talk to signal loyalty, or disloyalty.
2. We talk to bond with others.
3. We talk because we are not very self-aware and we need an audience if we are to learn our own thoughts or make up our minds. Clark Durant points to Hamlet.
4. We talk so people may judge us, leading to efficient sorting.
5. We talk to see who will leave the room.
6. We talk because we are restless, nervous, or bored. Speech may relieve anxiety, or give the pretense of doing so.
Of course it depends on context, but I’ll put information communication at no more than fifteen percent of our chatter.