For instance, his lecture on Dodge City (1939) starts off with a rough taxonomy of Westerns--how the 1930s movie Western grows out of gang/mob movies, how the Western provides several different responses to the Great Depression and the New Deal, as well as the old question of capital/labor/technology. Then Slotkin notes how Dodge City avoids almost all racial questions by being almost all about white outlaws and white law-men, with the one black servant being a comic character in the prologue. (Where his joke is basically, "I can't keep up with these modern times," which is why he's not around ever again.) Also: no Indians.
If you're really into genre and history, then it's really interesting to see how this Western thinks about the growth of civilization. (And also how it takes part in the 1930s "heroic Confederacy" myth: the good guys are all ex-Confeds, while the bad guys are Union. In fact, if you liked the scene in Casablanca where patriotic French out-sing the Germans, you might want to watch the Dodge City brawl where the heroic position is taken by the Confederates singing Dixie over the ex-Union men singing "Marching Through Georgia.")
But if you're interested in writing a Western, this sort of film school analysis has its limits. It's not bad to have an eye on questions of politics and personhood. (Are all your black characters minor and there for comedic effect? Maybe consider a revision.) But a lot of this stuff is post-hoc for a reason: You're not going to write something very interesting/lasting if you've got one eye on "how does my movie address this current legislation?"
(Actually, on my trip up to Chicago, I listened to this one Jack Benny show from 1946 where half of the jokes were so topical that I couldn't at all understand them. I'm not saying that you shouldn't write topical material and only write "universal" stuff, only that there's a definite lifespan for certain issues and stories in your entertainment.)
This sort of post-hoc critique can help you think through big issues, like your main character's plot arc (or lack thereof--in Dodge City, Errol Flynn's character becomes a sheriff and vows to bring law to Dodge City, but when we first see him, he's... bringing law to the wilderness). And it's always good to expand one's film vocabulary, I think. But there are limits to this sort of criticism if you're looking to write the thing being criticized.