> There is also the issue that they do only provide a minimum of security thanks to the "put the captcha on a porn site" method.

Only relevant to large sites. Like WAHa said, it raises the bar.

Besides, there are ways to prevent that too. Ensure that the referrer is always your site, and prevent any IP or block of IPs from requesting too many captchas, and their job becomes a lot harder.

If you make it hard enough, hopefully the undesirables will move on.

> They are also annoying the user constantly, infact they annoy me than even the spam messages do

Depends for what. You normally only see it once when signing up for an account somewhere. It's a tiny investment for a large collective return (which is steadily diminishing).