Captcha code are the worst thing in the world they are incredibly hard to read,robots can just crack thought them so they don't even their job right. spammers and cheater can just paste it to an other website and get the code and just keep pasting it their and bypass the entire thing in the first place.
my info came from.
http://contests.about.com/od/sweepstakeslaws/tp/How-Do-Cheaters-Crack-Ca...