That's stupid. They are the same story, the same spirit, told differently, for a different medium. They need to provide an experience apart from the books, so you don't even need to pick one of them up to get the story.
Not sure who said that (wasn't me though), but it's a tricky one. Of course they need to adhere to the core story and the characters (and not have, for example, Remus suddenly turn out to be a bad guy); but in a Hollywood movie there will have to be changes. Just putting everything from the book to screen will result in a film that's 6-8 hours long at LEAST. I agree with you that a lot of it is dumbed down, or aspects changed to the overall detriment of the story. I just don't agree with your statement that you can't judge the movies without reading the books. That's just silly, and should not be the case with any adaptation.