When talking to people who work for browser vendors about that use-case, I've heard phrases like "that's a really obscure use-case" and "No one is really doing art-direction".
This got me wondering — how big is that use-case? How many Web developers & designers are willing to go the extra mile, optimize their images (from a UI perspective), and adapt them so that they'd be a perfect fit to the layout they're in?
With the lack of solid data on the subject, I had to go get some :)
Arguably, one of the easiest ways for Web developers to implement art-direction today is to use picturefill — the framework that polyfills the picture element's syntax. So all I had to do is find sites using picturefill and see which ones use the framework for art-direction rather than simple resolution-switching.
I've used the WebDevData scripts to get a hold of Alexa's top 50K websites' HTML. Then I grepped through those HTML files to find pages that contain "data-picture" (the data attribute used by picturefill), downloaded the images and (manually) went through the results to find which sites art-direct their images. Not very scalable, but it works for a small amount of sites.
The results showed that 24% (7 out of 29) of the sites that use picturefill, use it to implement art-direction. While a larger sample would be better, this is a strong indication that the art-direction use-case is an important use-case for responsive images.
Embedding the Gist with the results: