I've always thought that it would be good to allow non-members to post in one topic, if they have any quick questions but do not want to sign up for the site just yet.
This means that:
-More visitors will come to the site if they have any questions as it would be quick for them to do so.
-More people will join the site (If their questions are answered quickly that is) as they like the feel of the place.
-TowersStreet will become more popular and will be pushed higher up in the Google Rankings, so even more visitors will visit the site!
Of course, this would have to be moderated a little bit more than usual, but I'm sure members who answer the questions could look over the topic and flag any inappropriate posts so that there is no hassle!