Mutable state is an optimization. Sometimes a powerful optimization with little loss of clarity, sometimes utterly ruinous to any hope of understanding the code. Usually it's between the two extremes. But I think the default should be the functional style. To reach for imperative tools immediately is usually a premature optimization.
It's not just an optimization. Functional style is often less clear and more confusing than using mutable state.
For example, I move my browser window. My browser models this as a Window object whose Position property is mutated to a new value. This is clear and easily understandable, in part because it matches the user's experience of moving a window.
A functional approach might be to construct a new window at the new position that shares some structure with the old window. This seems totally weird, and furthermore, it's hard to think of any benefit to keeping the stale, no-longer-onscreen window around, while it's easy to think of problems that might cause. We can pile more machinery on top (FRP, lenses, etc) but we only make the functional solution more complex.
(On that note, GUIs have got to be close to a worst case for functional programming.)
