Either is typically the sum-type equivalent of Pair, which is a tuple that's limited to 2 fields. I don't think Either is very interesting—I'd rather see a sum-type equivalent to tuples.
But Either also isn't a very good way to model Result, or any other sum type with 2 fields. I suspect you wouldn't model Point with a Pair<Float, Float> or a (Float, Float) most of the time either. point.0 or point.first is much less clear than point.x. Adding semantic names, which makes the abstraction less generic, is actually helpful.
(Similarly, you could replace Optional with typealias Optional<Value> = Either<Value, Void>, but I don't think that'd be a good idea)