The whole point about where the mutation happens and how the language remains pure in spite of it is not awfully relevant. The point is that Haskell does suppor

The whole point about where the mutation happens and how the language remains pure in spite of it is not awfully relevant. The point is that Haskell does support real, bona-fide mutation in its standard library (see for example its support for stateful IO arrays). So obviously mutation is useful, regardless of whether you're doing that mutation behind a pure interface.