It's not about the battery, it's about the portability and the display. A 13" laptop is too big to lug around for me, and the retina display would enhance readability for me (I tend to read a _lot_ on screen, but an iPad is simply not an option for the rest of my activities).
I'm willing to wager less than 1% of people who are full time MacBook Air users will convert their full time laptop use to a Surface Pro 3. And of that 1%, I'm guessing > 50% will be attracted to the Stylus/Sketching capabilities.
You missed his point: retina needs a lot of battery power, so a retina 11" would need a much bigger battery (or a large loss in power time), which goes against what the 11" is about
