touting polymorphism

Whether polymorphism is useful depends on what one wants to do. Often people would be better served with sum types (aka algebraic data types), pattern matching and exhaustiveness checks. Unfortunately, sum types haven't caught on nearly as much as product types (aka tuples/structs).
An interesting perspective is cast on the advantages and disadvantages of sum types and polymorphism respectively by the expression problem:
http://en.wikipedia.org/wiki/Expression_problem