I am a member of several Aussie boards which alllow guest posts, and it
does get abused. Some delinquent members will not log in, and then say what they please without regard for others. Ugly and unsociable, which is why I prefer TechIMO and don't frequent those boards much.
IMO there is no need for Guest posts - if you want to post, make yourself known. Just like if you come to my house and want to come in. Identify yourself, and take responsibility for what you say.