Well, I think forums work by "critical mass", so you'd want to make it as easy as possible if you're starting. I can't believe that if you were trying to start up a forum and have some of the costs paid for by ads that you'd want to make it any more difficult than necessary to join. On the forum I help out on, we have plenty (probably too many) volunteer moderators so spam isn't really a problem - it gets removed immediately. In fact we get very few spam posts, and we take turns removing the few sig spammers we get every day.
In any case, you're obviously trying to help people fight spam and promote yourself and your mods, so more power to ya. And if you have a site that's so valuable that people pay to join, that's great. But I guess we'll just have to disagree on whether paid registration would be right for everyone.
That's an interesting point of view. If you are just starting a forum, the spam may be manageable by conventional means such as captcha, Q&A, and old school moderation as you said. As you expand and get more traffic, even for a small-mid size forum, things get out of hand very fast. Factor in moderation time, negative impact on the brand's image, and ineffectiveness of conventional solutions on a larger scale; these things cost the administrator much more than the members would be paying to register, put together! Imagine how your volunteers could be used to spend their time and resources into creating positive change on your forums rather than destroying negative spam.
That said, members paying to register seems to be a recurring concern and I would just like to throw the idea out there that you can simply refund the fee. Thank you for inspiring this concept; maybe I will work it into the modification as a feature!!!
I work on making the modification right for as many people as possible!