Because it's a bit too error-prone: mut listOfLeftHandedOralHygienistsInKazakhstan = getThem() // several lines later listOfLeftHandedOralHygienistsIn

Because it's a bit too error-prone: mut listOfLeftHandedOralHygienistsInKazakhstan = getThem() // several lines later listOfLeftHandedOralHygienistsInKazahkstan = getThemAgain() You thought you were reassigning the mutable variable, but you actually created a different immutable variable. This can be prevented by having different operators for assignment and re-assignment. Some languages do that: in OCaml / F#, re-assignments use '<-' while declarations use 'let (mutable) x =' (they can't drop the 'let' because they use '=' as the Boolean equality operator).