Commenter David points out this New York Times piece from Mamet on why he wrote race that I missed. I think it’s revealing, particularly this graf:
I have never spent much time thinking about the themes of my plays, as, I have noticed, when an audience begins to talk about the play’s theme, it means the plot was no good. But my current play does have a theme, and that theme is race and the lies we tell each other on the subject.
I pointed out in my original post that individual lines worked much better than the play’s structure, and that the show might have been much better as a play about lawyers. Maybe Race would have been a better play if Mamet hadn’t suddenly decided to focus on theme, particularly one where his thoughts seem poorly-developed.