If you know them really well, voice is more than enough.
Video gets more info about how certain, cooperative, serious, how well listening etc people on the other side are. Also the relationship (boss/right hand etc) between speaker and non speaker on the other side gets more obvious.
Decisions should be made in writing, so everyone gets some peace and quiet to deliberate, and we don't change our minds because nobody remembers what the outcome was (sounds crazy but I have seen this happen).
Unless you're engaged in a high-stakes commercial negotiation, for example. The sales people in those are always on camera. The lawyers are routinely off. Part of that is a long, looooong history of negotiation via conference call, while part is wanting to be able to roll our eyes at the other team's posturing without being caught.
