You need to know what ordering the other user is seeing, otherwise dangerous misunderstandings could result, for example you reply "yes" but it's in reply to a

You need to know what ordering the other user is seeing, otherwise dangerous misunderstandings could result, for example you reply "yes" but it's in reply to a different message than the other side is seeing. If I see that another message has arrived and my message could be misunderstood, I can correct that by sending another message. But if I don't know what ordering the other side is seeing then I don't know if my message is ambiguous or not. The only way achieve that is to show a consistent ordering to both participants (or to force every message to be in reply to another message but that's too nerdy)