My decision of a vote of yes was mostly derived through Andrew's trains of thought. Since half of the registered members do not post, anyways, nor do the guests, what harm can be done by making it a requirement to register?
Even if they must register to view the topics, the hits will keep coming, will they not? To my knowledge, the hits received do not come from reading the posts, but simply connecting to the site itself. I may be wrong in this, but if that's the case, what could go wrong with mandatory registration?
More members, more referrals, and more preceding members. In any sense, with mandatory registration, the site can only become increasingly popular. We won't be losing any of the 100+ members currently registered just because other must register.