When you consider that the other options aren't free, this becomes even more true. A heavier or thicker case, plastic body, shittier display, or even something as seemingly insignificant as a bigger power brick to save a few hundred bucks? No thanks.
It looks like a competitive laptop, although I'd have to hold one in my hands to really know. Looking at the pictures I'm guessing that the trackpad would drive me crazy (I don't understand why other manufacturers have so much trouble here, although Vizio seems to have done a nice job with their new line of laptops in this department -- no separate buttons for one). I'd also be trading OS X for Linux (or Windows), would would be a pretty serious compromise for me[0].
Anyway, yes, the mind can justify whatever it wants, but I don't feel like I have to do a lot of work to justify spending an extra $200. That's substantially less than a day's gross pay if you've got a typical US software engineering job.
[0]: I'm no stranger to Linux. I ran it exclusively from 1999-2004, and I have a Linux dev desktop at work. Out of the box I strongly prefer OS X, if for nothing other than the consistency of keybindings across software (including third-party). I could (and have) customized my Linux config to be more consistent, although some things are not easily tweaked.
I'd also say that the Zenbook has a dual core i7 with 4MB L3 cache. The MBP has a quad core with 6MB L3.
I'd also say that the SSD on a UX32 looks like it's 128MB standard, half what comes standard on the base MBP Retina.
And finally that even Ubuntu/Mint have far more issues than OSX. Things like always reliably coming back from sleep modes. I still have issues with this on Ubuntu 12.04 on my desktop machine.
Look, i think the Zenbooks are beautiful and if all you have to spend on a machine is $1300 then I'd seriously consider it vs the 13" Air. I'd probably go Air because I have a lot of Apple ecosystem but an apple noob would probably go Zenbook.
But you can't compare it to a Retina 15" Macbook Pro. It's not like everybody that buys Apple just mindlessly forks-over an extra $900 for the brand name. The machine packs a punch.
These things in a 2010 MB Air, where you wouldn't expect it, were unparalleled. When I spend 10+hrs a day on a device, I need it to have flawless input devices, and the Apple products seem to deliver the best (OSX req'd for best integration).
I don't know about you, but that sound about parallel to me. For $700 less.
Last I checked bloatware was really an issue on most linux distros [Archlinux leaves you with a terminal prompt]. I'd recommend Ubuntu for new users, but as a Mac user you'd feel right at home. When you then take into consideration that performance of Wine (the free, open source Windows emulator available on linux) when running native Windows applications (Including high-graphic-demand games), it's a developer's dream.
Stickers...well there's precious little anyone can do about that.