I'm not from a Ruby/Lisp background, so I don't know how symbols are typically used, but the explanation of why symbols are required is to prevent collisions if two libraries chose to use the same property name; so given that, what is the point of the Symbol registry? I don't understand what the value of a symbol that is referenced by a string is, over simply using the string?
See `Symbol.hasInstance` and `Symbol.iterator`; take them as examples. Mapping an object to its iterator is going to be part of the spec and will be required at the language level (`for...of`). Before, if you looked at all of an objects' properties in SpiderMonkey, you will have noticed an `@@iterator` property. The spec could achieve the same thing as it's trying for with `Symbol.iterator` by just using a string-valued `@@iterator` property, but you still get the collision with anybody who would have used that name for something else or anyone looping over all the properties.
The gist is this: symbols are just a way to have non-strings that you can use for properties. There are also facilities to generate them in such a way that the symbol will never, ever collide with those generated by somebody else, but that's not their only purpose.
I think the idea is so that while symbols can prevent collisions, they can also make it impossible to set a value. The registry allows you to set the value by name, but by doing this you have to keep in mind that there might be a name collision and allows you to handle the collision. Before there was no way tell that a name collision even happened.
