Shoutbox

A smart spammer, or one who needs to earn a living from it, will avoid captcha's as much as possible, even if they are breakable.

Because captcha's also create an extra overhead, which means loss of time. Which means that they could make more money spamming an unprotected service than a protected one. Because for even a simple captcha, it still takes extra resources and time to bypass it (being done automated or not)...

But no, ofcourse captcha's aren't 100% suitable for everything (eg: once sign up on a forum, you can usually make as many posts as you want), but for certain things it is effective (eg: password guessing - MS uses a captcha after x failed attempts, making brute forcing them a whole lot harder and more time and resource consuming).