Kayak does this too, and even Travelocity did it a while back (although their version may have been killed). There are other sites as well than use a similar (or identical) metric.
Why do you think these features haven't taken off (that's an assumption)? Maybe it's the map format? I've really enjoyed the curated approach that theflightdeal has taken and now hipmunk. As I said in another comment, I wish Kayak let you filter by cost per mile. The really great deals get lost in all the other deals.
If.you're a fan of Kayak explore you might find http://tripcommon.com interesting - essentially aggregating deals like that but for multiple people coming from different origins
