Albeit a necessary evil.
Captchas, are those funky graphics that pop up and ask you to key in the text contained in the picture. the purpose of Captchas is to prevent spam by making sure that a human being is doing the typing. the Captchas are intentionally made so that OCR software will have a hard time reading the text, but the normal human eye can easily read them.
That's the problem, there are more than a few of us out here who don't have normal eyes. Some captchas rely on low degrees of contrast between the text foreground colors and the picture background colors. These sort of captchas can be impossible for me to read. They make me feel like I did when I was kid at the eye doctor, and they showed me those funny color blind tests with the dots arranged in circles. There may have been a number in those dots, but I sure couldn't see them. And some CAPTCHAs may have text in them but the chance of me reading the text correctly is pretty low.
Captchas are necessary, but don't use them unless you need them. There have been more than one occassion when I've made 4 or 5 attempts to get past the captcha to make a post, and decided it just isn't worth it.