Somebody could sign up as a member and then try to screen-scrape and republish everything somewhere, so members should assume that whatever they say will eventually be public. And the membership might still decide that read-only for non-members is the right thing to do-- I agree that it could be a good way to attract new members, if the quality of conversation is high.
This could be something that should be asked from the members, at the forum at least. I don't have anything against a read-only forum but I don't know about other members. I think the members should decide something like this.