Was going to get the rMBP 13 but no discrete graphics is BS. Currently my 2009 MacBook pro has better graphics than a 2012 machine! That is nuts, what was apple thinking?
Call this a hunch, but I'd hazard a guess that Apple have a little more experience of knowing how to build a 13" retina MBP than you do (particularly after you've proved you can't even write a post correctly).
No disrespect, but don't you think Apple spent considerable time figuring out if they could get discrete graphics in to the 13"? My assumption is that they simply couldn't add discrete *and* keep it running cool.
Besides, you don't need discrete for most tasks. I'd say for 85% of my 15" rMBP use, integrated graphics is in action. The system only switches to discrete (for me) when editing images in Photoshop or apps that require core graphics. However, I bet the switch isn't essential - LittleSnapper doesn't *need* discrete to run, even though discrete is switched on. Indeed, I spoke with the developers about this and they are looking at a way around this switch as they agree it's not required.
So, add discrete to your 13" rMBP and there's likely to be a pay-off and Apple decided against this. They'd rather have a longer battery and a cooler machine and I think most users would agree.