doubleofive said:

I think the screenwriters did the right thing by looking at every plot point and asked themselves "Does this affect Harry or his battle against Voldemort?"

I think this is linked with what you posted next: no foreshadowing.

The kids's's's''''s interaction with Snape seemed to be the first thing on the chopping block for most of the Book -> movie conversions. And then given Snape's role in the finale... I think the movies so shortchanged Snape that it fundamentally changes how the viewer reacts to him at the end of the story.