It is worth noting that supplementary material has been added from other Tolkien writings to make the three films. Of course things have to be changed for film, Tolkien himself said that. While I don't agree with all the changes made, it in no way 'ruins' any of Tolkien's stories for me
The reason why I enjoy works from Tolkien's so much universe is that the stories were written to occupy a world already created, which was in turn created to explain the languages Tolkien had created. As a historian and ancient linguist I like to get lost in the detail that some find too heavy. I understand how they feel, I stopped reading the LOTR after the first few chapters as a teenager and it took it up again later in life.
That being said, the Hobbit is a lovely book that is easily read by anyone, since it was supposed to be a children's book from the Tolkien universe. I think PJ has brought in other material to flesh out the story and make it appeal more to adults and fans of the LOTR.
Part of the joy of reading ancient epic and sagas is looking at the different versions and how they changed as they were passed through time. I think PJ and his writing team have earned their place in Tolkien history with their re-telling of the story. They certainly attracted younger generations towards the works of Tolkien, which, regardless of their medium is never a bad thing.
P.S. I still can't wait to see a war between dwarves, elves, men, orcs and wargs!