I think riding a dragon is too high fantasy for ES. And just because you can kill something doesn't mean you can make it your mount; I think dragons would rather die. Plus, having a flying mount would make traveling way to easy.
So riding dragons are too high fantasy but talking humanoid lizards and cats, shooting bolts of lightning fom your hands, and ancient wizards who live in giant mushrooms aren't? Kind of seems like an odd standard to me.
Still, I don't particularly want riding dragons either, and the reason is because dragons are supposed to be a credible threat to the world, not to mention intelligent beings that even get their own language, they shouldn't be degraded to the level of glorified horses, just because they're big and can fly doesn't mean they're willing to let you ride them.