What if you want a clean API? What if you want the users of your code to be able to instantiate an instance and inspect that object in the console, seeing only the public methods and not having them get lost in a sea of private stuff?
I've been burnt in the past with using someone else's javascript code and finding out that something I needed to modify they hadn't deemed public for some weird reason.
Forking the code isn't the best pattern for code reuse.