Similarly to what happens on Tasks, where messages can be removed by the poster and Phabricator admins, we should be able to delete messages sent to Conpherence rooms. I suspect Phabricator roots can do so via DB, but doing so via web interface would be easier and more transparent.
Usages: disclosure of private information removal, spam removal, innapropriate content removal, etc.
(yes I know that contents can be delivered via email, etc. - the same happens with Tasks and other Phabricator objects so I don't consider that a valid objection not to do this and permanently store unnaceptable content on chat rooms)