I'm not saying that this is the original poster's reason, but I can
think of a few reasons why you'd want to try to write a program to defeat
CAPTCHA with good intentions.

   1) It's a form of Turing test. If you're into AI, that alone should
drive you to try to defeat it.
   2) It's one of the easier Turing tests. If you're a pretty good
programmer, and you want to get your feet wet into the world of AI, this
might be a good challenge (certainly easier than the traditional test of
making a chatterbot which can hold a meaningful conversation).
   3) You realize that you're not the smartest programmer in the world, and
yet you think you can defeat CAPTCHA. If you can defeat CAPTCHA, that means
someone else can probably defeat CAPTCHA too. You don't know if the intent
of that other person is pure, but you know your intents are pure. You can't
stop "evildoers" from solvign CAPTCHA, but what you can do is solve it
yourself, and then publish your algorithm and an explanation of how you
solved it so that a better CAPTCHA can be constructed.

   - Oliver