Does An Adaptation Have To Be Faithful? And If So, To What?

There’s been a lot of hubbub about how The Hobbit isn’t a real adaptation of Tolkien’s work – the light-hearted tone of the original work is submerged under a lot of Serious Battle Scenes, there’s a lot of shoehorned-in references to Lord of the Rings to lend if gravitas, and why the fuck does it have to be three hours long?
Yet Jay Lake posted an interesting link today: Dislike Peter Jackson’s The Hobbit? Then You Don’t Know Tolkien.  This essay basically says that Peter Jackson’s knowledge of Tolkien is far superior to yours, and the only reason that The Hobbit is viewed as a light-hearted adventure is because Bilbo and company were ignorant of all the dark machinations going on around them.  Jackson, he argues, is actually putting The Hobbit into the proper context.
Which is interesting.  Because both sides are correct: Peter Jackson is more accurate at depicting all the political and personal pressures that led Gandalf to yank Bilbo out of his comfy little hobbit-hole.  But the tone that Jackson takes is not what most people come away from the book with.  It’s as though someone decided to adapt The Shining like this:

Full disclosure: I’ve seen The Hobbit twice, and I quite liked it.  But I don’t think an adaptation has any obligation to the source material except to be entertaining.  Planet of the Apes is nothing like the book, but it’s good.  Same with The Birds.  So if The Hobbit is a good movie on its own, then I don’t really care how faithful it was – and though the Hobbit is a flawed movie, I think it suffers both from people who wanted it to be LotR 2.0 (which it isn’t) and The Hobbit: the Book (which it really, really isn’t).
Which is not to say that I don’t get upset by bad adaptations.  Tim Burton fucked over two of my favorite movies.  But I’m not mad because they diverged from the source material, but because they diverged from the things I was most entertained by in the original films.  And I reacted in the way I always do to something I’m sure I won’t enjoy; I didn’t watch it.  (Having heard my friends go on at length about the differences, though, I feel fairly justified.)
As it is, I don’t think fidelity is a truism – there have been some mighty faithful adaptations that fell flat, because to turn a book into a film, you have to play to each medium’s strengths.  The Hobbit isn’t really what Tolkien wrote.  But is it good on its own, if you had no prior expectations?  I think it’s decent – not the slam-dunk of Lord of the Rings, but a B, not too bad.  And I think that’s a much nicer way to see it without getting bogged down in the geek wars over what kind of Tolkien-geekery should have made it into the film.
After all, this is the same Tolkien who wrote Tom Bombadil.  I’m quite happy to see his stuff wind up on the cutting-room floor.