Good article on why a flat ui might be good at: http://alistapart.com/article/material-honesty-on-the-web
Crux of argument can be found in the statement: "Even lighting effects like drop shadows, form shadows, specular highlights, and reflections are dishonest because there’s no light source inside a digital screen that’s manufacturing these lighting effects." The article then puts forward the position that honesty to material creates more timeless design.