CAPTCHA isn't just a matter of protecting your site. One of the most evil attacks nowadays is "Distributed Spam Distraction", where you spam your victim with thousands of emails per second so an important email (e.g., fraudulent purchases) gets lost in the noise.

How do you do this in a world with decent spam filters? By using the victim's email to sign up for real services so they get hit with a welcome email. Because these are real services, spam filter won't catch it. This can only be done with services that have sign up forms that are easily automated.

The most evil thing here is your email is crippled even after the attack is over because these real companies will keep sending you newsletter and it's impossible to unsubscribe to them all.