Folks, if you think about it, why would Elphaba NEED to fly during "Defying Gravity?"
She sings "It's meeeeeee" and raises up to the sky.
If she were to fly side to side, it would take away from the WONDERFUL song that she sings. It would be distracting.
"The Spectacle has, indeed, an emotional attraction of its own, but, of all the parts, it is the least artistic, and connected least with the art of poetry. For the power of Tragedy, we may be sure, is felt even apart from representation and actors. Besides, the production of spectacular effects depends more on the art of the stage machinist than on that of the poet."
--Aristotle