you lose a bit of quality, too. It might up the ratio towards quality, but you'll definitely lose some comments that the author doesn't feel like jumping through hoops to send through.
example : captcha/captcha-like services.
I've written well-thought out responses and comments on various forums and communities , and finally when asked to identify all the buses or traffic lights I quit the process.
Why? Is it because the comment isn't worth while? Is it because some moral repositioning enticed me to not to go through with the comment?
No.
Clicking every bus increases friction, I may not be all that motivated to begin with -- this friction pushes me over the edge and I close the tab. The captcha/whatever process may have succeeded in the job of fighting spam, but it also lost my comment to the ether.
I contend that gatekeeper systems like pop-quizzing the reader, and to an extent captcha-likes, are not necessarily a good gauge for content quality but rather of poster motivation.
Captcha doesn't stop spammers, it sets the bar higher -- requiring spammers to harbor a larger motivation in order to create the systems needed to circumvent captcha-likes.
A pop quiz would not hinder someone filled with vehement hate from posting a comment -- they'd skim the text, cherry pick the answer, and proceed to post the hateful or incorrect comment.
A community filled with people who post due to a large motivation seems like a bad thing.