I think only registred users should write to the shoutbox, because an open shoutbox incites to ask there before searching in the forum or to look at the wiki if their question has already an answer.
If users need to register, probably they look at 'read first' and see how to post. Especially if they search for a topic before posting, it may help, even not ensure, that they post with a topic different to 'tell me', 'please tell me', need help'...