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.
