umm, the Glance screen (like the lock screen) is mostly white space. Only 10% of the image is covered up by content. The Start Screen is completely the opposite. When we say we can't see it, we don't mean we have no idea it's there, we mean that the subject matter in the image is barely visible or discernable. Therefore, absolutely pointless.
A background image adds clutter to the screen. Period. The entire philosophy of Windows Phone / Metro is minimalism. Subway signage being a key inspiration. When you add background images to anything, you are adding another layer of visuals the eye has to separate from other items. This adds stress to the eye when trying to find what its looking for. Its even worse off you have an image based live tile that would then be on a background image as well. This isn't myth, it's usability.
We are arguing against it because we recognize and value the design philosophy of Metro. We respect it's principles and don't want it's integrity ruined by a bunch of whiney users who think their Ferrari should be able to go off roading.
I can't even count the number of Android users that have a busy background image to the point you can't see or read the app icons on the screen. Microsoft should not allow users to ruin Windows Phone the same way. Sometimes you need to keep the knife away from the user so they don't cut themselves.