The actual narration of the story, its editing (involving his original and corrected versions in parts), and its passing on to other hobbits are all a part of the story. The process of writing the story tells some of the story in this case.
I know it is not typical to have a large framing in general but there are some precedents where this worked well, in my opinion:
The Princess Bride
The Never Ending Story
Labyrinth
Fried Green Tomatoes
I wouldn't just stick a frame onto any movie to add to the screen time of it, but if like these other stories (and like the story as it is told across several books) the narration itself plays a part in the story's telling, then by all means, include it, I say! Not to mention that this is a quick and dirty way to get a lot of relevant appendix material in - if done carefully and artfully. It's not the only way - flashbacks work for those parts that happen before The Hobbit. But a frame can be used in many more creative ways and is great for making bridges to other things if that is part of what they are trying for here.