The maintainability requirement ultimately flows from the "one rule" as well. It's not as urgent as "runs without crashing every five minutes" but it's a valid concern. If everything goes well the company grows organically, turns the corner, and Chuck's code is gradually superseded by the process weenies that come in when the company gets large.
If you hold on to unmaintainable code too long, it will be impossible to add features, maintain performance, port to new systems or do things that customers want and need.
So I agree that it's important; but not that it needs to be its own rule. If you go on a witch-hunt for Chuck before he is "in season" you'll bag Chuck but the customer will get away.