The movies will stand or fall on their own. The movies don't have to precisely track the books, no matter what the purists think. If he adds new material (which he CLEARLY is doing, since there's all kinds of stuff that happen but are completely off-scene in the book but are being shown in the movie), then that's fine as long as the tone and tenor is consistent with Tolkien's world.
Changes only bother me when they literally make no sense. The Witch King breaking Gandalf's staff and blowing him off his horse in the extended version of RotK REALLY pisses me off. It makes no sense and seems to show that the Witch King is clearly more powerful than Gandalf. WTF is that about?
But otherwise, judge the movies for what they are. Jackson has earned a fair bit of latitude I think, given past success, and I hope he makes three really great, enjoyable movies.
"I love signature blocks on the Internet. I get to put whatever the hell I want in quotes, pick a pretend author, and bang, it's like he really said it." George Washington