Golly, I love this film. No greater insight to offer, really. Just love this damn film.
Sure, the exorcism tacked on at the end by the studio is a little bit out of step, who cares? It's psychedeleriffic.
And here's a swell little nugget about William Peter Blatty from wikipedia:
Blatty appeared on Groucho Marx's quiz show, "You Bet Your Life" (1950), posing as an Arab sheik with so many wives that he could not recall how many he had. Groucho was completely taken in. When Blatty revealed that it was a hoax after a couple of minutes, he told Groucho that he did it because George Fenneman had said that Groucho was an expert at spotting phonies. Groucho replied, "That is incorrect, because I've had Fenneman in my employ now for 14 years." Blatty won $10,000 on the show, and when Groucho asked what he intended to do with the money, Blatty replied that he was going to take a year off from his job and write a novel. That novel was "The Exorcist".