Yeah, I'm aware of this. But I don't think this prevents the germanwings outcome (which was, indeed, orchestrated by the "completely insane pilot" you mention).
The person plotting to commandeer the cockpit always has the upper hand, because they'll be the first to act. It's hard to defend against a sucker punch or a knife from first-class.
You could even argue that the two-in-the-cockpit rule is less secure, because it introduces one additional person who could pull off a germanwings scenario into the impenetrable control room. This is the same concern raised by the pilot quoted in this article (http://www.smh.com.au/business/aviation/germanwings-australi...):
"It exposes the cockpit to more security risks than the isolated case of a homicidal pilot," he said. "I think flight crew are a better judge of fellow pilots' mental state. Now I have to judge the cabin crew member's mental state too before leaving them in the cockpit with access to things like the crash axe."
Originally I had a long aside in the article to deal with this point, but I axed it because I don't think it changes the core argument. Unfortunately Medium seems to have removed footnoting capabilities. But it's still a valid note. Thanks.