Many of those UIs end up being pretty clunky– the home interface for example. Sure, you get a little more information (good for webpages and maps as shown) but at what cost? If they were to modify the aspect ratio to expand the screen while conforming to their retina spec, it would make more sense just to transition to 16:10(8:5). At the very least, they could do it in a way that makes it more consistent with desktop applications.
The best thing the 3:2 ratio has going for it is making portrait orientation feel just a tad friendlier than a reporter's notebook, at least when viewing horizontal media.