Spam protection with obscurity is different from security through obscurity. Spam protection through obscurity is fairly prevalent and reasonable.
Unless you're taking a blood sample from every user, you are going to have spam that you can't catch.
It's a certainty that the spam you can't define is the spam you can't catch. The "I know it when I see it" test doesn't scale.
If you could tell which comments are insightful / relevant / interesting / unique it wouldn't matter which ones were produced by humans and which ones by algorithms.
Likewise humans can often create spam by hand - daft comments / contributions that hurt your site but come from legitimate humans.