Besides the Air's battery life and computing power which both aren't enough for Retina display, I believe there's no need in it. Apple won't do rMBA in several next years, since they position rMBPs as the pro's tools. The majority of simple users (who are the target audience for MBAs) don't really need the Retina display in MBA, because it's screen resolution is already crisp enough. Since most of them are extremely happy with their current Airs, I think simple switch to IPS screens with the same resolution will make customers as happy as it can get.