The chat buffer interface can be inconvenient for some purposes. As you can't use RET to insert a newline (use C-j for that), writing a longer message can be painful. Also, it is not possible to include a subject in the message, or send the message to multiple recipients.
These features are implemented by the message composing tool. Type M-x jabber-compose to start it. In the buffer that comes up, you can specify recipients, enter a subject, and type your message.