What are the destructive aspects of being able to ride a dragon, art-wise?
The way the dragons have been implemented (at least from descriptions and trailers) seems relatively subtle and well-managed, compared to what could have happened. They all look like they belong in the world, and they aren't absurdly present. They can talk, but there's very little evidence of Ghost Rider Syndrome. In other words, the designers didn't just say "What would every pre-adolescent boy find super-awesome about dragons?" and add that to the game.
Riding dragons is an inherently ridiculous thing to do. It looks silly and over-the-top. It obliterates any sense of the initial restrained design sense that the developers offered up.
How to Train Your Dragon was an awesome movie, but it doesn't fit in with the overall aesthetic of this game.
Like I said, I know I'm not being asked to play it. I just don't like seeing people take a really good design and run it into the ground (or in this case, put it in the air).
Edited for some of the new stuff: If it
does make an appearance in the main quest, I'm willing to bet that the player will not be in control of the journey.