True. That is why I think the default should be that it behaves like it does now, but that advanced developers can tap into the details and take advantage of it.
I'm needing this info so that I can develop more device-independently than the current CSS standards allow, so I'd be a user who would benefit from this and not shoot himself in the foot like what you described.
I legitimately believe that if I had ultimate control of pixels in DOM (we already do in WebGL), along with knowing hardware pixel density, that I'd be able to make amazingly device-independent things.
For example, with a CSS pixel to hardware pixel ratio of 1, along with knowing pixel density, I'd know the exact size of the screen in the real world, the world that the user lives in. Then I could decide how big things to be on screen. For example, if I detect a device that's like 2.5" by 4.5" or something, I can appropriately fit a sliding menu on the side that is 2" wide, and be confident that it will look the same on all devices where that menu fits (and by "look the same" I mean "be the same exact real-world dimensions in the real world of each user (we all exist in one real world, and the UI would exist in that real world too, not scaled behind some virtual glass)).