Seems to me that they are an easy way out and band aid solution to a problem that could be solved in other ways. I have never implemented a captcha field on a website and do not intend to in the future, but I am curious when I see Google, Yahoo and eBay all using them. Surely there are better solutions to the problem.
As a user I find them incredibly frustrating even when they have been implemented well like reCaptcha and I am unlikely to convert if a site requires me to complete it. Surely there are many other users out there that will think the same.
Captcha are also not infallible with some spammers offering incentives for people to provide captcha answers in desktop apps.
With so many other options for traffic filtering/limiting is it just through laziness that we have ended up with so many out there or is there a reason I am missing?