I struggled on this point for a time. I nearly held out, but was decided to get a 13' MBA (4GB, 128GB Storage). Its turned out to be a fantastic purchase and I wouldn't upgrade in the foreseeable future.
Assuming the weight of the 13' MBPr scales in relation to the 15' MBPr, one could expect a 21% decrease in weight. That would mean the 13' MBPr would weigh 3.58 pounds compared to the air at 2.96 pounds. While the .622 pounds may not seem like a lot, its really worth it if one travels a lot. The design and function of the MPBr is to be a portable desktop. If one is traveling a lot, going to class, or doing tasks that aren't high end video/photo editing, then the Air is way better. The portability factor, incredibly fast charge time, and great battery life and cost are huge selling factors.
Lets be candid, if one is going to upgrade, its going to be for the screen. For me, it was easier to just wait out on the screen. The Air screen is actually quite nice (especially since I upgraded from an early 2008 MBP) and will serves most of my media needs.
Doing a little research on retina displays has led me to believe that within an upgrade cycle or two, the Air line will come standard with retina display. A lot of this has to do with the intel chip upgrades. From what I can gather, the Haswell chip set (followed by Broadwell) are going to offer quite a substantial boost for graphics processing over Ivy (essentially doubling the graphics processing capability). Additionally, Haswell is designed for use in ultraportable devices like the Air making it all the more enticing. Speculation seems to be leaning toward a retina display Air with the release of Haswell and/or Broadwell chips. This means one will probably be able to purchase a retina display air within two years.
Given the estimated cost of the 13' MBPr will be at least $1499 (possibly higher), I would rather wait for a generation or two as the MBP and Airs will come standard with retina display and other advancements (such as cheaper SSD and Haswell/Broadwell chips). One is essentially paying a premium for having access to the screen now. Future iterations of the retina display will be cheaper and featured on models people want to own. Look to the evolution and pricing of the Air to see how quickly Apple drops the price on a product and how much more one gets if one waits.
Apple has set the standard for the industry moving forward with the retina display on a laptop. The rest of the industry will eventually catch up and Apple will start offering more products at reduced prices that have retina displays. At this point, one is paying for for having access now which carriers a rather larger premium. For me, its not worth an upgrade because I want to purchase an ultraportable with retina display. The new 13' MBPr isn't designed as an ultraportable so its not worth for me to upgrade yet.
In the mean time, if one wants to have access to a retina display or HD quality media and isn't a dedicated video/photo editor on the go, get a New iPad or Apple TV. The retina display on the new iPad is "better" than the display on the MBPr in term of pixel density. Apple TV streams 1080p iTunes content and other media content providers such as Netflix already offer 1080p streaming with Apple TV.