Good manners in a negotiation? Of course.
Your counterparty is more likely to make concessions to you if they like you.
Moreover, a study by Robert Bies and Joseph Moag found that negotiators are more likely to accept terrible outcomes in a negotiation if the news is communicated in a fair and polite manner.
Sometimes it’s more about the “How” than the “What”. Respect, honesty and rapport go a long way in all types of consensus-building – in negotiations, conflict resolution and facilitation.