Possibly the most annoying thing I've heard from people defending certain fantasy movies is the argument that, since it's fantasy, who cares if it's realistic? After all, the whole thing is fake anyways. In other words, a character can defy the laws of gravity, but it's OK because, hey, it's fantasy!
That's just ridiculous. If, in the movie, the laws of gravity are present and intact, then no one should be able to violate them, no matter how amazing he or she happens to be. But how do we know that the laws of gravity apply in the movie? Well, do the characters fall down when they die? The law of gravity is therefore intact; and anything contradicting it, without a better excuse than the awesomeness of the character, has no business being in the movie.
This certainly goes in our novels as well as in our movies. We can't establish a rule (implicitly or explicitly) and then break it willy-nilly. If we do it once, then our readers might forgive it as an oversight. Twice, and they begin to wonder what's going on. Multiple times, and they start to nod their heads in boredom. What's the point of a story that breaks its own rules just for the sake of excitement or convenience?
I think that the people who make the argument of 'anything goes' are those who don't get fantasy. They'll watch/read it once in a while just for entertainment, but they don't understand people who really like it. So when people like myself complain about Thorin and Co. in The Desolation of Smaug being chased around by a dragon that breathes fire and can fly, and this same dragon being unable to even singe their beards, they just don't get that a story must stay true to its own rules or else it becomes laughable. Or maybe it's because the viewing public has been so dumbed down that a stupid chase scene that stretches the movie out way beyond what it should have been is more interesting than what should have happened: a tense, witty scene in the dark with Smaug and invisible Bilbo. Because who wants dialogue when we can have a dragon covered in molten gold just for the fun of it?
That's the way we'll get people to regard fantasy as a legitimate form of literature or film.