It's because extending reading beyond books -- into movies, soundtracks, games and toys -- gets kids reading.
Of course, it all has to start and end with the book. Which is where parents come in. You make sure the child doesn't become sidetracked and, like, skip the book.
Let the movie draw them into the book. But then you have to show them how much more exciting the book is than the movie.
And it will be, because only the book can engage the child's imagination fully. The reader can add detail and let the adventure fly. The book can become whatever the child wants it to be. The movie, on the other hand, gives it all to them on a plate - so of course it's limited.
It's like... my son would rather have cookies and pizza than vegetables or fruit. But it's my job to make sure he eats right. And afterwards, he always feels better. And as an adult, I know he's going to be a vegetable-eater.
OK, enough of that metaphor.
My point is, that however you get your child interested in a good book is fine. Movies and other book extensions (Potter Puppet Pals on the Internet, for example) that get them interested in the book, are fine. And they work.
We're turnin' kids into readers, here.
Yeah, the metaphor definitely couldn't go on much longer or you'd find out that I'm actually pretty terrible at getting my son to eat his veggies. Some days I am forced to count ketchup, pizza sauce and relish as "veggies."