I'm not keen on making an option on how the subject behaves. An option for under 2% utilisation seems like a waste. If we can't agree that the subject is optional (or redundant) then I'd say it'll have to stay as mandatory. Personally I have nothing against having a subject, I just don't see why we have to Re:iterate it adnauseum!
As for SVG, yes they are Scaleable Vector Graphics. Yes they are, underneath, XML. But everything's something under the hood. They're a great way of having a dynamically sized image. Inside the XML you might say "there's a line that's 100 pixels in length" but when you render you can make it twice as large or twice as small, and it'll draw the line as either 200 or 50 pixels.
It doesn't resize the image, it recalculates how the entire image is rendered, and I think that's the power of SVGs.
They're more orientated around geometrical shapes and so do have some limitations on what you can draw with them, but they're very powerful (and normally very efficient).
If you're on the responsive site you're seeing one already up the top left -- the impossible cube. Visit
http://responsive.rpol.net/images/impossible_cube.svg, the image will be huge (but it's scaleable, remember) but view the source and you'll see only 39 lines of XML. That's the entire image and I can make it 100 pixels wide or 10,000 without any loss in fidelity.