The headline for this Newsweek piece was pretty silly, since they never answered the question they asked... 'United 93': Is This Film Necessary?:

But the movie's strength—its wrenching realism—is also its limitation. Greengrass gives us the surface and sensation of reality. As a picture of behavior in extremity, it rings true. But without context, psychology, politics or contemplative distance, you may wonder what, exactly, this re-creation illuminates. 'United 93' isn't exploitation, but what is it? On some basic level, it can't help but be a thrill ride, albeit one that leaves you somber and drained. Whether you need that ride is a question you'll have to answer yourself.

Contemplative distance? Context? The problem with those things is that they distort what really happened. I haven't seen the film yet, but if it truly is shot in documentary form, without trying to change history, then it would be a very worthwhile film. It will remind people of what actually happened, and let people interpret those facts for themselves, without someone else providing what they deem to be "needed context".