The problem I think lies in the change in Denethor between the book and the film.
Book Denethor, if you remember, is very like Faramir in personality "the blood of Westernesse runs almost pure in their veins." In Boromir it does not. So Faramir has a close affinity with his pa, despite the latter's preference for the robust man of action Boromir.
The Denethor of the film did not have any of the decayed nobility of the book character, which I agree does make Faramir's devotion rather odd. Furthermore, in the EE of the film, they saw fit to have Gandalf say that Denethor was like Boromir and unlike Faramir, in direct contradiction to what Tolkien said in the book. Way to go, PJ...
