I agree that some deviations are necessary. I didn't understand the scene with Bellatrix and Fenrir Greyback attacking the Burrow, though. What do you suppose was the point of that?
I wondered about that, too. Perhaps, since Hedwig was in the house at the time, that's how they are going to take care of her departure?
A lot of movies are pitched at children (or adults) with a mental age of 8. Some folks in the audience haven't read the book, didn't pay attention to the first 6 scenes of the movie, and haven't decided yet if Harry Potter is supposed to be the good guy or not. So, the director has to "help these viewers" along. How to do so?
Have the Death Eaters show up at someone's house and set the whole thing on fire for no reason at all. This establishes that the Weasleys are good people being victimized by bad people. And it establishes that the Death Eaters are (wait for it...) actually bad guys.
I'm sure I'm over-stating this, but I swear some directors really try too hard to make sure everyone "gets it".
1. It give Bellatrix more screen time. Always a good thing, as she's delightfully fun to dislike.
2. It emphasizes once again Harry's rashness and the ease in which he can be lured into a trap. So little of this movie is about Harry, it's nice to have that point hammered home.
3. It shows that no-one, not even the Weasleys, are safe from the Death Eaters at large. They cut a lot of the material in the book about students whose families are suffering under the attacks, so this illustrates the issue without the distractions from the main storyline.
4. To a lesser extent, it reinforces the whole harry/Ginny thing in that it's not just one-sided from Harry's side.