Well, I wouldn't really say that. Some (okay, many) alterations had to be made so that the films would be shorter and understandable. Some of
my favorite parts from the book(Old Forest, Scouring of the Shire) were cut from the movies, and it did at first really torque me off. But it doesn't make a huge difference to the whole story. The two tales are basically side plots that have little real effect on the story. (Except for Merry's sword, as
Saphy pointed out earlier.) They would just add time, and in the end, it would make the movies less desirable for someone new to Tolkien
But if they had put every detail into the movies, we'd be watching some 20 odd hours rather than 11.5. I wouldn't really mind, but that's the main reason that they had to cut so much of it. Thank God for the Extended Editions!