I got the platinum mumble extreme edition of the Two Towers and listened to the director's commentary during the parts where the plot diverged radically from the book. A recurring phrase they'd use was "a slight deviation". And, placed in the context of the longer cut of movie, it all made much more sense. For example, once they had decided to move Shelob to the third movie, they had to create some sort of conflict for Frodo and Sam to work their way out of, thus the changes to Faromir. Likewise, the changes made to the battle at Helm's Deep were intended to fill in some backstory on the historical alliance between men and elves without dorking the main story line around too much. (In the extended movie, Fangorn forest has more of a role in the battle as well.)