It creates a clever excuse not to unite our faith and deeds. It allows people to shirk responsibility under the guise of diversity. Rather, God would have us unite our faith and works, so that faith becomes visible. Verse 18 teaches that, faith being invisible, it is impossible to see it if it is not made visible by deeds. Our lives works show our faith.
Talk is cheap, in other words.
What we do, reveals whether or not we truly love the Lord. James will then employ several examples from the OT to illustrate his point. Like good preaching, he makes several points in principle, and then illustrates them. Note that he is not afraid to use Abraham and Rahab as examples. The OT was just as relevant as it ever was.