When a message arrives, jabber-message-default-message will usurp the minibuffer to show "Message from XYZ" (with lots of blank space to the right). Fair enough, but why not use the full space and do something more useful with it?
jabber-message-fancy-message will add as much of the message text as possible to the line, concatenating multiple lines into one, trimming the result to prevent the minibuffer from popping up to show more than one line, and applying some fancy highlighting in the process.
This way, when someone messages you something short like "ok" (or even something slightly longer), you don't have to switch to the chat buffer right away.
Again, whether to make it the default I leave up to you and if you think it doesn't belong in the code base, please put it on the emacs wiki instead.