I have seen it, and I agree that the background of Faramir greatly improved the movie. However, that doesn't change the fact that they altered the plot.
Actually, now that you mention it, I remember it being explained on one of the extras why they did this. Once they decided to push Cirith Ungol to the third film, they realised that Frodo and Sam needed some obstacles, including a climax, in the second. If Faramir had been as accommodating and friendly as he was in the book, there would have been some dramatic let-down, as Frodo and Sam would not have faced any challenges to speak of.