What is the unique contribution of the book of Esther to the canon of Scripture?

This is a book describing the Jews who have remained behind in Persia, when they have been freed (and commanded by God) to return to their own land. This is not a book that describes faith and obedience, but disobedience. It shows God's providential care of His people in spite of themselves. The absence of the name of God (other than in some hidden way), along with other disturbing characteristics has to be explained some way, and making Mordecai and Esther look like model saints is not the way, in my opinion.