Bilbo's Encounter with the Black Emperors

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Anyone who's actually read Tolkien's original masterpiece knows that Peter Jackson has taken certain liberties. Some of them more irritating than others... I thought I'd begin with a rather minor one, but a divergence all the same. The main issue I take with this revision, is its needlessness. The original scene was changed, however, in the books (and the movie) it isn't of extreme significance, so I must ask, why change it at all?

First of, they are supposed to be black. The butterflies that is. In the movie, for one reason or another, they are blue- turquoise if you're a colour snob. This minor change, doesn't really bother me, I can imagine black butterflies, from a distance, may appear too much akin to moths.

However, take a look at this quote, spoken by Bilbo in the movie.

I can see a lake... and a river. And the lonely mountain. We're almost there! Can you hear me? I know which way to go.  - Bilbo Baggins

This is then followed by the spider scene, which actually occurs at a slightly later time in the book- but I digress.

Now look at this quote from the book.

He looked at the 'black emperors' for a long time, and enjoyed the feel of the breeze in his hair and on his face; but at length the cries of the dwarves, who were now simply stamping with impatience below, reminded him of his real business. It was no good. Gaze as much as he might, he could see no end to the trees and the leaves in any direction.

Do you see the discrepancy? In the book, Bilbo doesn't spot anything, the only thing he gets out of his climb, is the feeling of invigorating wind. In the movie, he apparently sees the Lonely Mountain, and is now planning to lead the dwarves henceforth. As the following part of the story, has nothing to do with reaching their goal, but rather getting captured by spiders and then the great Elvenking, why was it changed? It doesn't contribute to the plot, and is thus, in my mind, a completely unnecessary deviance from the book.