There are lots of ways to render beautiful, unobtrusive UIs that are difficult to emulate. For instance, a Retina-detail polished metal button would be readable and look good but it would be non-trivial to recreate (unlike our current login dialogs that could practically be recreated in MS Paint). It would be especially secure if it included personal information, e.g. an etched-in-metal appearance of your full name.
While I agree that today's phone UIs are boring, I don't think replacing them with photorealistic, "extremely-difficult-to-render" UIs is a good idea -- nor would that protect against imitation by malicious app publishers.
Usability would go straight out the window. Every menu in every app would become an assault on the eyes, or at least an incredible nuisance. UI elements should be clear and consistent, and should stay out of the way.