Why to avoid using reCaptcha(stop spam , read books.)

I have a newly installed phpBB3 forum , and in over 2 weeks I gained at least 60 bots , just by using reCaptcha , and while they claim its harder to guess then any CAPTCHA I am sure its easier to guess then any. The best captcha you can get is making your own. Like a question , or someone to type in something from your forum display ... that's impossible to be guessed by bots , since they don't read , they just guess. vBulletin has a nice feature implemented like that already. I just wonder when phpBB will. For now the only solution I found is to delete the bots manually daily , and to use the implemented GD CAPTCHA , instead of reCaptcha.

The bots are cunning nowadays , they can even mail confirm , guess captchas , some of them has avatars , profiles , just like any of you. Also there are paid sites who pay people to spam others forums. Such as http://www.decaptcher.com , so be very careful , and use hard to guess captchas , and hire moderators :P