It has many uses such as preventing standard automated software from filling out a form with some assurance that a human is at the keyboard.
The most vehement haters of captcha tend to be those that want to perform click fraud or worse.
I fear that the alternative is to take such forms and services off line.
Bob
Is it really necessary for those captcha letters and numbers to be so twisted and warped that half the time you get them wrong, because that 7 was really a 1, or maybe an upside down L? ***!!? Since they're graphics anyway, is it necessary to warp them at all? There has to be a better way. I think whoever designs them is just F'in with us because their job is designing F'in captchas.

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic