Sure, but let's assume that the program has more than one thread and that another thread could still be using the old value. In that case, an imperative program might be required to copy the whole structure or sleep until the existing users are done, which is often less efficient and is always more complicated.

If it's ok to support only a single concurrent user of the value, then a mutable structure is indeed more efficient. Even in Haskell we have mutable structures that can be used for such purposes.

The interesting question to me is, what should be the default? I think there is a good argument that it should be the safer, simpler immutable structures.

"Simpler". If you're only single-threaded, the mutable structures are simpler.

If you're multi-threaded, you have to choose: Do I go with immutable, which means that the other thread gets something safe? It also means that the other thread gets something stale (out of date), which can have its own consequences.

Or do I use a lock or mutex, which means that I have to place guards in all the right places, which is error-prone and can break in weird ways if I miss one?

Or do I use mutable data without guards, and just let it crash? (Because it's probably going to crash if I do that...)