Personally I don't see why I would go back to using classes. IMHO, classic OOP doesn't fit to modern JS.
I guess it's good to have the option though.
ES6 classes aren't "classic OOP", they're syntax sugar for regular prototypal inheritance that JavaScript has had forever.
The point is moving mixin handling to userland (and eventually some sort of standard?), if I understand correctly. It's not really React's concern.
See also: Minimal API Surface Area http://www.youtube.com/watch?v=4anAwXYqLG8