(Spoilers for The Hobbit 1 and 2, as well as the entirety of the book, to follow)
For all of the flack that Peter Jackson's Hobbit trilogy is getting, it's actually either harmless to or is improving on the narrative, and here's why:
1) The Dwarves' Expedition makes more sense in the films
In the book, it doesn't really make sense that they require a burglar. Think about it: there are two scenarios: A) Smaug is dead, in which case a burglar is not needed, the dwarves could just go into the mountain with no issues, they have the only key after all. B) Smaug is alive, in which case Bilbo is pretty much useless, what's their best case scenario? Bilbo sneaking in, filling his pockets and sneaking out? Then repeating until something goes wrong and he's killed?
In the films, Bilbo is essential because of the changes made by the film makers, but most people don't seem to have picked up on this, even the CinemaSins people on Youtube, who make it their mission to over-analyze movies didn't notice it, calling the scene that establishes this fact redundant to the plot. In the films, it's already been made clear that Bilbo is there to retrieve the Arkenstone, because in this version of the story, the Arkenstone will give Thorin the power to unite all the dwarves, march on the mountain and slay Smaug. This is never said in the books, and Thorin only wants the Arkenstone for personal reasons. Therefore, in the films, Bilbo's presence and the dwarves' plans make more sense.
2) The alterations provide more plot development and more themes.
Who has character development in the books? Bilbo and... no one else. Yes there's Thorin, who in the books is unjustifiably greedy and stubborn before overcoming this in the end, but that's a very rushed subplot, if it even qualifies as one.
In the films we're seeing development in Gandalf, Thorin, Bard, Tauriel, Legolas, Bombur, Fili, Kili, Azog, it's happening all over the show. You could not have this without adding some things to the story because the books are based entirely around Bilbo, and one Hobbit simply cannot support a trilogy even if he is the namesake. You may buy your ticket back to middle earth, but you must barter passage with forgiveness of some added subplots to allow the world the dimensionality that made the first trilogy so great.
3) Bard's backstory is better than in the book
In the book, Smaug's weak point is a spot on his jewel encrusted underbelly where a gemstone has fallen off. It's a complete coincidence that this has happened and it is therefore a complete coincidence that Smaug is killed at all. In the films however, Bard's ancestor, Gideon, knocks a scale from Smaug's underside but fails to kill him, meaning Bard must use the advantage his family has left him to restore his family name, which is much more poetic than Bard getting a lucky shot at a lucky target.
4) The dwarves actually fight their mortal enemy
The dwarves fight Smaug, using Erebor itself no less. This is better than the books, where they leave the men of the lake to take on all the danger while they watch from the mountain doing diddly-squat and then take the moral high ground later on. There were 8 of them and they managed to hold their own against a dragon AND deal an ironic (auric?) revenge that drove him from their homeland. I can think of no better way to improve that segment of the book.
5) The book and film stories mesh perfectly
The main argument against these new plot-lines is always the same "they weren't in the book!" Well just give me one good reason why that means they didn't really happen in the story? I mentioned how the book follows Bilbo, but at the same time it also uses him as the eyes and ears for the reader. Now, let us run through the changes to the book's story, in chronological order, shall we?
In the book, as in the film, Bilbo didn't know the story of Smaug's taking of Erebor, he heard the song about it, but that was about all, so it doesn't contradict the changes made.
The Meeting at Bree
When Thorin and Gandalf meet at the beginning of The Desolation of Smaug to discuss why a burglar is required, Bilbo never knew of the meeting, so it is conceivable that it did occur in Tolkien's original story, and in fact Tolkien wrote a similar event as a short story after the book was published.
The Dwarves are pursued by Orc riders led by Azog the Defiler. Now, if we hadn't been told of the tragic battle of Moria earlier on (which also did not happen in the book but was described by Tolkien himself), then we wouldn't even have ascribed Azog any significance, he'd just be an Orc, part of the great goblin's forces, he could have been there, but Bilbo saw him as no more than an Orc, and hence so did the readers.
Tauriel/Kili/Legolas love triangle
People also disliked the injection of a new character, a returning character, and a new romantic subplot because all three were not chronicled in the original book. But here's the thing: Bilbo sees none of that.
Bilbo is not present when the Elven guard capture the Dwarves, he isn't there when Tauriel talks to Kili, he isn't there at Lake Town when Legolas is killing Orcs and he isn't there when Tauriel cures Kili of the Morgul blade's curse.
All of the accusations of the film being a failure because it deviates from the story of the novel are rendered fully and completely useless by the simple fact that Bilbo, and hence the narrative of the original Hobbit text, did not intersect with them, Bilbo didn't see it, hence WE didn't see it. And that's the best argument I can give in favour of the movies deviating in no large way from the source material.
Thank you for reading.