That particular image is simply an example. It's not supposed to be a background.
Let's say for example you're building a website about bridges. You want to span the top of your page with a picture of a bridge where the bridge supports fall at 20% and 80% blending perfectly into your 3 column layout. You then position:absolute your title text on that bridge and it looks pretty good on your 1024x768 screen. You want that image to take the top 20% of the screen, no matter what size screen. By specifying height:20% for the image and width:100% I would expect you would have an image that would span the screen and only take up the top 20% of the screen. I could then visit your page from a 1958x768 widescreen notebook or a cell phone and still have the graphical fluff look right. You don't care about the aspect ratio of the image, because you've designed it so it looks good no matter how badly you stretch or compress it.
You can't use a background image because it doesn't get resized at all.
The point is, the spec says that it does not keep the aspect ratio if both width and height are specified (with no exeption for percents).