[ajuc]

It seems they can be used as a little safer enums, like :keywords in clojure. It can come handy if you want to add method to object that nobody will overwrite by accident.

But clojure was designed around :keywords, while js was designed around strings as property names. Adding another possibility now complicates everything.

I don't know if I like it.

[nailer]

I think you'd add methods nobody can overwrite using Object. defineProperty(), which is an ES5 thing.