Captcha just sucks, I've used it on my site and it does nothing, determined bots will get past it one way or another and you also have to acknowledge the fact it's fucking annoying to get past them as a human, the only ones I've found remotely bearable are the numbered ones. Like DRM it's an over-marketed heap of junk that claims to do something but in reality it does nothing to combat the thing it was designed to stop and can be abused in all sorts of ways.
Captchas can be really annoying if they are impossible to do but the more impossible they are the more likely it will stop bots
, unless the bot uses a service which asks a random person to solve it and give it the answer. 