A forum progresses into a club imho by bringing a community together by offering membership; that membership should be able to attract new members to join and current members to continue with their subscription with incentives... these may include discounts off parts, visits to car shows, merchandise etc. etc.
Without it, it really is just a DB full of words that have no real "ownership" - if that's the case then it should be paid for via advertising, otherwise someONE foots the bill or there's a constant rampage of sponsorship requests.