The statement comes from experience, extended stuff is bad just bad for you. jQuery and others are dropping the use of hasOwnProperty. So if you extend stuff and include one of these libs, they will break. The devs just don't care anymore. Also, there are people who are even more against it that really say NEVER EVER extend them, not even for the sake of backwards compatability.
Also, my guide isn't even need the controversity of some of the statements that Crockford has made ;) In the end it's all advice, I'm not forcing you to write code this way.
In fact, I like it when people question the stuff I write, the worst thing that can happen is that people pick one book/guide whatever and just follow it 100% without ever thinking on their own.
Again, it's not 'bad for you'. You don't NOT use a hammer because it doesn't work well with screws. It works great on nails, so again, context is everything. Supplying best practices that include the words 'always' or 'never', often fall quite short and don't serve people well.
Also, my guide isn't even need the controversity of some of the statements that Crockford has made ;) In the end it's all advice, I'm not forcing you to write code this way.
In fact, I like it when people question the stuff I write, the worst thing that can happen is that people pick one book/guide whatever and just follow it 100% without ever thinking on their own.