Why can't I directly modify a component's state, really?

Question

I understand that React tutorials and documentation warn in no uncertain terms that state should not be directly mutated and that everything should go through setState

Answer 1

My current understanding is based on this and this answer:

IF you don't use shouldComponentUpdate or any other lifecycle methods (like componentWillReceiveProps, componentWillUpdate, and componentDidUpdate) where you compare the old and new props/state

THEN

its fine to mutate state and then call setState(), otherwise it is not fine.