Functional programming is not just programming with closures. It's about "referential transparency". Referential transparency permits higher-level mathematical reasoning, which leads to safer/better program composition, optimisations, caching/reuse and parallelisation/concurrency. If you are willing to concede mutable-state and other uncontrolled effects, there is much to be gained.
You might mean "enforced referential transparency" by having immutable variables and no I/O. Otherwise most languages allow you to have pure functions and nothing forces you to mutate state.
