So what do you get when you read a file, or when a file is uploaded to your web server? What happens if you write a function that accepts a string as a parameter, but haven't noticed that you're implicitly assuming the string is utf8? (e.g. a function that formats one string using another - if the encodings are different you'll end up with a string that's invalid for either encoding, no?)

The distinction between a string with one encoding and a string with another is subtle but vitally important - exactly the sort of thing a type system should take care of.