1: http://www.anandtech.com/show/5996/how-the-retina-display-ma...
Also, look at it this way: with the diminishing returns of resolutions above 300 dpi, is it really a big deal that you won't get the absolute best use of the iPhone 6+'s 400 dpi? Seems like a pretty reasonable trade-off to use that extra resolution in a way that makes resolution-independence easier for developers.
The other reason why I think this might be a good thing is since 1080p is more of a standard than the other resolutions they're playing with, which might help unify things moving forward.
Tying the size and density of your display to software needs-- especially points that are arbitrary to begin with (as they are based on the original iPhone) seems silly and outdated.
My first MPB was one of the "high resolution" ones that ran at 1680x1050 and I loved it. So when I first got my 15" retina MBP was miffed that now it felt like I was running a 1440x900, just with double (well, quadruple) pixel density. So I opened scaling preferences and set it so the retina screen would "emulate" a 1680x1050 display. It was terrible. Blurry and SLOW. Good lord it was slow. And impossible to do any design work on. I went back to normal 2x scale.
So sure, this kind of scaling has been available for a long time but the reality is most people don't scale their MPB retina displays to anything other than 2x.