1) Casting Fantasia IS a marketing device for the show. It doesn't matter how she got the part, her name is a marketable name and why wouldn't they use it to boost ticket sales? You think they're gonna keep her a secret?
2) Unless you know Fantasia personally, you have no right to judge Fantasia's "worthiness" to "be on Broadway." You may not like her, you might not think she is talented, but what does that have to do with her deserving to perform? If you want to show producers that you don't like her or "stunt casting" then don't buy a ticket. Just know that in many cases, your not buying a ticket won't matter overall. The public and fanbase will come anyway.
3) "Stunt casting" can be a blessing. It has a negative attachment to it because of it's history - many times the big names that come have never even showed an interest in the theatre. We've had Usher, Jacob Young, Julia Roberts, Denzel Washington, Toni Braxton, Tara Conner (almost), and all these American Idol folk - plus the dozens of others I am blanking out on.
One can only hope that they are RIGHT for the part and can act FOR THE STAGE (which is different than film/TV). If the producers can use a name to raise ticket sales, then hopefully it's for a show that is still alive and hasn't had its day. If stunt casting could have saved CAROLINE OR CHANGE, power to them. If stunt casting is being used for a show such as PHANTOM, then I say let it go.
"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