The interest of subscribing to CP Passport and the advisability of having pay-only fora are two different things IMHO.

Pay-only fora is a bad idea, always IMHO; First of all, the life of a forum depends on the number of people who frequent it. More people come and read, more chances there are somebody will answer some question: I've posted something once in a forum, and it took one month for someone to answer; The same question posted elsewhere was answered in less than a day. Need less to say I never returned to the first place again. Why would I?

Second, a forum doesn't cost much (some bandwidth), and is a good way to create customer loyalty. People taking their habits of chatting there will be coming back, and having customers flock around a store is always good for a store.