Well, Apple's font rendering technology (Quartz) makes fonts less sharp on the screen than Microsoft's rendering technology (ClearType). This lack of sharpness is much more noticeable on low resolution screens. The only way for Apple to make text more readable on the screen without changing its rendering technology is to adopt retina displays. So, a retina display is more important on Macs than on Windows computers.
Putting an IPS screen on a MacBook Air, therefore, won't solve the problems related to the blurriness of the fonts.
That's why retina displays are so important for Apple and that's why they are a requirement for Apple computers (but not necessarily for Windows computers).
I agree that it should be a well-balanced machine. But I don't think the retina display would unbalance it. The iPad has a retina display and it is even thinner and lighter than the MBA. In fact, the weight and thickness gained by the iPad when the retina display was added was almost negligible. And Samsung has already showcased a 2560x1440 resolution ultrabook, showing that this is perfectly possible.
I don't see why a retina display would compromise the design principles. Retina displays are becoming even thinner and more energy efficient. Battery life will be much improved with Haswell processors, so more room for battery. The prices of retina displays will come down too; they are probably not too expensive at this point, since Apple has managed to put a retina display on a US$ 499 iPad in the beginning of the year.
I am not saying that the MBA should get a retina display right now. But it will definitely get one soon, within a year, perhaps.
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Really? I get seriously annoyed by Apple's rendering technology. I can't just zoom out text as much as I do under Windows due to the lack of sharpness of fonts. This problem would be gone with a retina display.