The mirror in question may be the looking glass that peers into Alice's topsy-turvy Wonderland, perhaps, but it produces an image none the less. The mirror is the event horizon known as the sale. On one side is the world of marketing, and on the other is the world of user experience design.
The practice of user experience design is, of course, nothing like the practice of marketing. But then the practice of calculating the square root of a number is nothing like the practice of calculating its square. And yes, the fundamental ideals of UX are different from those of marketing, to be sure. And if you're a UX professional, you might balk at the idea of your hard, noble, earnest work being compared to the slimy tactics of vacuous marketroids. But to paraphrase one Dr. Frank N. Furter, to the balkers of such a comparison, I didn't make it for you.
The human mind is a sucker for symmetry. The imaginary path between two concepts, that which is bisected by the symmetrical axis, forms an extremely strong analogy. And just like the analogical relationship between our left hand and our right, symmetrical needs not mean identical. So when the question is raised—what is the value of user experience design—a concept scarcely understood by those in positions of great economic and political power, you can answer with a question about a concept that is: what is the value of marketing?
That's easy. Marketing is everything. Marketing is the difference between being rich and being bankrupt, between fame and obscurity. Without marketing, no effort matters. Marketing is so important that business executives will throw untold resources at even the daffiest schemes without batting an eyelash. In many cases, a much greater quantity than the resources they allocate to their products.
Ah.
If marketing is whatever you need to do to book a sale, then user experience design is whatever you need to do once that sale is booked. Why do you care if you've already got their money? Because everybody knows that the best marketing is a happy customer, who buys from you again and again, who tells their friends about your product, who talks you up in prestigious publications. With business models shifting away wholesale from the singular transaction of goods to ongoing service relationships, the repeat customer—the fan—has never been more essential.
By this point, our executive counterpart should be sold, and will immediately proceed to order his marketing department to spawn a user experience arm. Stop him.
Remind him that a mirror image is not a subset, that UX is no more a part of marketing than his right hand is a part of his left. The goals of UX are divergent from those of marketing to the point of being antithetical. The methods are different. The culture is different. But the outcome, and therefore the value, is the same.