The point of using immutable in this case is to prevent mutating the state by accident as a side-effect of another action.

I know that react-redux will only rerender if things changed through a dispatch (in fact, everything consuming redux.subsrcibe will not notice a side-effect change), but those things happen if you do not use some kind of immutability and especially if they have no immediate effect, it's a nightmare to debug.

Read the article, this is discussed in the first few paragraphs.