I don't think any sane person would use mutable strings considering the benefits you get by having an immutable string type by default. I have always said that if I were to make my own scheme (my language of choice) I the mutability of strings (and pairs) is one of the first things I would change.

Or were you thinking of anything else? I am not saying that a mutable string type should not be provided, just that it should not be the default one.