You'd be surprised. With automated posting tools, the cost of probing many different sites and posting massive amounts of spam (in an effort to raise search engine rankings) is very low indeed. I've run several small forums, and without fail, eventually they'd be overrun with spam and I'd have to either 1) install authentication or 2) install a captcha.

I agree that captchas are user-hostile. But, when weighed against the alternative - a forum or comments section so filled with spam and nonsense as to be unusable - captchas are by far the lesser of two evils.