Yeah, we're kind of chasing our tail here.
Sure, if you're super vigilant and never, ever, make any mistakes, you never slip, you never have a bad day, then you don't have to worry about anything.
It would be nice, though, if a slight mistake didn't have to have catastrophic consequences, because you would be protected by a second layer of security. That's what a secure environment really is about. You wouldn't want to fly a plane in which all of the safety systems were interrelated and dependant on one single switch, which you could trip by mistake. Sure, if you're super-vigilant and never trip that switch, then all will be fine, but that's not the point. Redundancy is the single most important factor when it comes to safety and security.