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.
That sounds plausible. I feel much better about it now. ;)
And at the end, he stands dumbly by while someone he has never trusted takes a horrible action.
I'm not disagreeing with you, just the way they portrayed the character. The first action was within character, the second was not.