Captchas are generally for thwarting spam; why would you want to plague your users with captchas while they're logged in? For logged in users I'd simply use the "Min time between posts" forum permission to stop mass posts.

I only require captchas for forums that allow guests (unregistered users) to post; your users who validate emails should be seen as verified as not abusive (since you can submit to stop forum spam and ban if they become abusive).

Now, if I had a limited use group for new members (where new members are restricted from using several different features) I could use the recaptcha as a method of validating that they're not going to abuse utilizing those features in the future, then once they graduate after having 10-20 posts (via the post count groups) they'd be able to be a regular member and not have the captcha.