I think we have to consider that with the proposed scenario we more-or-less increase the amount of pixels to render fivefold.
regardless of whether the image is scaled on the client in css and how well they are reduced in filesize by JPEG compression, this will increase load times, and page responsiveness as it uses up cycles on the smartphone.
not everyone can afford the latest flagship device … that’s also one of the reasons we render the mobile version on the server instead of the client.
Also, bigger images will require more space on the server.
… just saying 
After seeing the latest responsive changes coming in 0.6 I don’t consider this such a big deal any more.
(though in the future, maybe even in 0.6, we need a setting to not load the mobile view at all :))