I'm telling you, with scientifically proven facts, that the GPUs (yes, both of them) in the rMBP and the non-retina 15" MBP are both capable of pushing 1680x1050 IN ADDITION TO 2560x1440 on two different Thunderbolt displays at once. If you do the math, that's over 2 million more pixels being pushed than on an rMBP using only its internal display. The GPU doesn't care about pixel density; it only cares about how many pixels it is driving. Again, as evidence, I'll point to a non-retina 15" MacBook Pro (let alone a 2012 non-retina 13" MacBook Pro) being able to drive its own internal display in addition to two external 2560x1440 displays and not breaking the kind of sweat you and Anandtech are reporting about. That being said, the only thing different in the equation of the lone rMBP is the software.
Apple software engineers have NOT been working on this for YEARS. If they have, I'd like to see proof of it. There has only been signs of work on HiDPI support in OS X in the last year, if that long. Since 10.7.3, if memory serves. Also, no I'm not in agreement that Apple won't release a 13" rMBP without a discrete GPU. They will absolutely release that machine without a discrete GPU BECAUSE IT DOESN'T NEED IT NOR DOES IT HAVE THE ROOM FOR IT! The problems your machine is having right now has got to be software simply because the hardware is only pushing pixels, and mind you, a fewer amount of pixels than the supported maximum of the same hardware running on different Macs (namely the non-retina MacBook Pros).
Before replying to this with nonsense about how I haven't read your threads or or the Anandtech article (neither of which are true) or about how I'm an overzealous inexperienced Apple fanboy, I entreat you to actually disprove, with facts and logic of your own, what I've stated here.
The standard MBP doesn't make sense when compared to the rMBP if AND ONLY IF you only want SSDs and built-in Ethernet, FireWire, and an Optical drive have no importance to you whatsoever...otherwise, it's a stupid buy as only 2% of the Mac software out there even takes advantage of that screen to begin with. The rest of the software out there looks like crap and will continue to do so until updated. Also, with the problems that you are now complaining about, it's obvious that this is the worst Mac I could buy for my money next to a current generation iMac.