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I have a slightly different view on movies made from books. Personally, I love the Lord of the Rings movies, but no movie could ever really do the books justice. That's the first step in making a good movie from a book. Realizing that you can never truly do the book justice. When I go to see a movie that's made from a book I don't want to see a movie of the book. I want to see a movie that's interesting in its own right. There are some movies that do this. Bridge to Terabithia, for instance, and Lord of the Rings. What you need to stay true to is the spirit of the book. Most movies made from books fail miserably at this, such as Narnia and The Golden Compass. Ug. You can't make a movie of the book. That's silly. If I wanted the book I'd read it. But make a movie biased off the book that, while it's interesting in its own right, stays true to the most important parts of the book.

As for giving lines from one character to another, some of those lines are important to the book, but while translating a book to movie form a lot of things must be changed. For instance, while Tom Bombadil was undoubtedly one of the best characters in Lord of the Rings, he did absolutely nothing to move the story along. When trying to fit as much story into a 3 hour movie as Lord of the Rings contains it's impossible to put anything in that isn't absolutely essential to the story, so Tom had to go. However, some of his lines were given to the character Treebeard as a tribute to Tom in the extended version. You may also recognize Old Man Willow in that sequence. Having that part helped to bring the movies a little closer to Tolkien's vision, even if they couldn't be put in in their original form. Unlike books, movies have time constraints. If they didn't, I'd agree with you and say that they should fully develop every character that appears in a book. But quite frankly, they just can't do that. So they focus on the main ones because they're the most important.