I think the point is these often are upvoted questions[1] with thought out answers. In fairness, I would argue the question I just referenced doesn't "belong" on SO as it's pretty clearly based in opinion. But if I Google "simple django rich text editor" and this is the first result, I would argue it does bring value to SO by bringing me onto the site.
Having strict moderation standards is SO's business. I may or may not agree, but it seems to work for them.
Leaving closed questions, particularly those without answers, on their site sucking up google juice and deceptively appearing in my search results is scummy. If they don't want it on their site, they should ask google not to crawl it. And that goes squared when the question doesn't have an answer.
[1] http://stackoverflow.com/questions/4674609/looking-for-a-ric... (not my find, posted in another comment on this thread)