MSDN has this, and I have never seen a comment there that actually provided value to a reader of the page. It makes even less sense on MDN, because MDN is a wiki; if you think something is missing, you can get it added.
Except that a lot of the time those user contributed notes are bad code that no one should ever be copy-pasting. I wish php.net had some sort of curation for those.
While the idea seems great in theory, I doubt Mozilla has the manpower to moderate it. The volume of comments would be astonishing, think Stack Overflow's Javascript tag – there are countless questions that have been answered time and again. Sorting and moderating them takes an insane amount of man-hours.
Just for reference, two years ago, during the Mozilla Work Week, one of the main problem that surfaced was that there were just a couple of people reviewing all new pages that were created and hunting for spam/illegal content
