Mutability is a feature, of course. Lots of useful algorithms need mutable data structures (e.g., dynamic programming, unification, etc.), and have no counterpa

Mutability is a feature, of course. Lots of useful algorithms need mutable data structures (e.g., dynamic programming, unification, etc.), and have no counterparts that rely solely on immutable ones. However, totally unconstrained mutability is definitely a shortcoming. The ability to look at a type signature and be immediately able to tell what might or might not change is a big win for maintainability, especially in big projects. By the way, constraining mutability doesn't require a purely functional language. The ML family (Standard ML, OCaml) allows all expressions to have arbitrary side effects (when evaluated), but all values other than reference cells are immutable.