Now you can claim a bad DM might mess it up because he's not constrained but then we're assuming a bad DM when it comes to a loose framework but a good DM (who follows procedure and agendas and etc.) when it comes to a game like BitD
You see, the main difference is, even an utterly mediocre GM can grab BitD, just follow the rules and get pretty decent results. Just following the rules of D&D can lead to a great game remembered for centuries or a game so naughty word everyone within a mile from the table would get hanged without a trial by the role-playing secret police. Or anything inbetween.
A good, experienced game master can run a game with nothing more than a sheet of paper, a pen and two asscheeks, sure. That's how I start my design process, and it works great to me. But can we count on a good, experienced game master when designing rules? Of course not.
Then, there's another thing worth discussing. You can't know what you don't know.
When you already are familiar with the genre, know the tropes and stuff, that's one thing. You can emulate it to some extent, even if you end up putting more work into it than necessary.
But what if you
don't know the thing the game is about? To hell with newschool, let's talk oldschool. I know pretty much nothing about caving, but when I use, say, Veins of the Earth, I get a claustrophobic cave-dwelling experience that I wouldn't be able to get on my own -- again, because I can't know what I don't know.