It's been discussed here on BWW SOOO many times. I, myself, think I created two topics on the subject.
Let's just get Ahrens & Flaherty on the phone and see what they have to say...
"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