> I often find refactoring in the spirit of functionalisation more powerful, because then what I do becomes less about implementing a specific piece of logic, and more about creating a robust library of combinators that can be puzzled together to implement the desired business logic.

The problem here is that your state is almost always transient and non-inspectable. Almost all the complexity in any non-trivial program is in the state, whereas the transformations are usually trivial and things like combinators don't really matter one way or another.