We're not talking mobile vs. desktop, we're talking responsive layout controlled by media queries, which can't be determined until the code hits the browser and

We're not talking mobile vs. desktop, we're talking responsive layout controlled by media queries, which can't be determined until the code hits the browser and your js (Modernizr or similar) measures the window. UA measures the screen size, not the window size. Not to mention that in theory, you would only load one size, but changing the orientation of a device or resizing your desktop browser window can trigger another breakpoint and essentially hide your ad, making the whole exercise pointless. And yes, the main point is always act in fear of Google, until they announce responsive ads.