I already abort a lot of comments I've written to post on forums, Reddit, HN, etc. Sometimes multiple paragraphs of written and re-written text. Usually my thinking is "someone will inevitably misconstrue my intent and start a dispute" - the chance of that often seems higher than the chance of someone finding my comment useful. So, I figure "what's the point?"

Adding a gateway, like your example with the CAPTCHA, would only reduce that chance of my posting further.

But as you indicated, it is a small barrier to a motivated, angry, agenda/conspiracy-fueled commenter.