For performance reasons mutability in Scala is often necessary. It was a financial shop and many things were too slow to just fold over. Closures are slow, and copying is slow. A lot of what we were doing was adding up and multiplying numbers. The difference in performance between, say, a while loop that adds up a variable in 1 million classes and a fold is so stark.
Moreover it’s not just about performance. Right now I’m programming a bittorrent client in Scala with Akka and it would incredibly difficult to do without mutation. And the complexity overhead that Haskell requires for mutation just isn’t worth it in a lot of cases.
I’m pretty good at determining if mutation doesn’t bleed into the outer scope and most of the time don’t want to pay the complexity overhead in actually proving. Sometimes you do want that of course, but not most of the time.
Also I don't see how ST would especially slow you down if you really want to write pure function with mutability under the hood? It's an interface you have to learn, but the benefit is you _prove_ your mutations are locally-scoped.