Haskell's Either isn't intended to be used solely as Left = Err and Right = Ok. It's supposed to work as an abstract concept of "one thing or another", and the language designers have avoided choices that would canonicalize it that way, such as introducing a MonadFail (Either String) instance.

One example of why you would buck the trend would be to use of Left = Ok and Right = Err for a series of "retry" computations. Say you have a few functions of type Either a String, which all get progressively slower, but which work on progressively more advanced input. An example might be a series of regex engines, each of which run slower but handle more and more advanced lookahead syntax. You can then write something like:

    makeRegex :: String -> Either Regex ParseError
    makeRegex regexExpression = makeBasicRegex regexExpression >> makeLookaheadRegex regexExpression

This will return a Regex using a simple parser if it can, and if that function fails (by returning a Right value!) it will go onto the next function and try that.