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An interesting discussion on the discrepancies between the book and the film in the Japanese BBS "2ch": http://book.2ch.net/test/read.cgi/sf/1078558932/l50
An anonymous poster claims that, while Tolkien's english "translation" of LOTR is based on the Red Book of Westmarch, PJ's films are based on another manuscript which was written from Gondorian point of view. This explains many discrepancies, such as the omission of the "Scouring" and Bombadil, Boromir's glorified depiction, the stress on the Aragorn's internal conflict, etc. Probably the hypothetical author disliked Denethor and twisted the facts, or maybe Pipin respected the Steward and glorified him.
(Update) I found a very similar argument here: http://www.markshea.blogspot.com/2002_12_01_markshea_archive.html