With CAPTCHA, I'm not so concerned about spam. Trolls would probably be a bigger problem. Since guest trolls would not need to register an account, they could easily post all kinds of annoying messages. More anonymity and lower barrier to posting attracts more troll behaviors.
I could be wrong; maybe there aren't many lurking trolls out there. You could try putting the guest section up, and if it becomes a problem, you could take it down.