Importance of Captcha in Forum Sites

There are a lot of forum sites that get a lot of spam from new users and there really are some good way to combat that without risking popularity. One way of doing this is by using captcha for new posters. That way a bot can't post in the forum and a real person will get discouraged by having to read captchas and entering them.
This feature is already in use on major forums and people starting out have to realize that even though it might give them something to do [deleting spammers posts and banning], the spam posts make a forum look trashy.
One or two posts every once in a while are fine and easily removed, but when there's a regular daily problem, that's when a forum site can quickly lose popularity.

Here is an example of captcha (you've seen it before)