Why is it "worst"? It is a great concept. Easy to observe in people. Easy to predict weaknesses if you know the type. Of course it is an Achilles' heel.
Is this some kind of "Fe opinion", that everyone should be able to overcome their weaknesses? Seems a little too idealistic. Especially with the knowledge of types.
Yes, it can be worked upon, and you do it by adapting and learning how to behave.
If you are interested in development, then I think Gulenko has some stuff on it, and also von Franz in "Lectures on Jung's typology", but she's mostly dealing with the inferior (suggestive) function.