We were still getting a lot of spam to the forums. A dozen posts a day and sometimes fourty or more over the weekend.
I thought we needed to try something, and this option was immediately available.
Since implementing, we haven't had the bursts of 20 or more spams with pornography in succession like we did before... we can probably create another user group in the forum for confirmed active users and have the CAPTCHA not show for you and others that we know won't be using it for spam. I'll see what we can get set up.