User can create an interesting situation by registering username XWikiGuest

Since there is no document called XWikiGuest there is no means of preventing registration of such a user. After registration, actual guests have the same access as the user called XWikiGuest. I'm not classifying this as a security issue since a user may simply share their password if they wish to achieve this end.
However I think we ought to create a place holder document called XWikiGuest to improve constancy.