Considering that all of last years models with an HD3000 can power a Thunderbolt Display (2560x1440) with no problems, and this is just 2560x1600 I can't really imagine it having any trouble at all. HD4000 is a much better GPU than its predecessor.
Come to think of it, I run dual 1920x1080 displays (effectively 3840x2160) without a hiccup on my 2011 Mini with the HD3000.
I don't disagree that the price is a little hefty, but all this whining about the iGPU is a bunch of fuss over nothing. Besides that, it's the first generation of a new product. The price will probably fall in line with the current MBP next year or the year after. The original MBA was introduced at a much higher price tag as well.
13" MBP hasn't ever had a dGPU, anybody who's wanted that has always needed to jump up to the 15". No one should be surprised this hasn't changed.
2d content doesn't require much of a GPU. My 2008 air could drive a 2560x1600 monitor without too much trouble either - at least with 2d content. Does that mean Apple should've never upgraded past the 9400M found in the 2008 air? No.
Dual 1920x1080 displays is not effectively 3840x2160. It's effectively 3840x1080 or 1920x2160. You'd need 4 1080p displays to have 3840x2160 pixels.
The problem is the equipment of the 13" rMBP doesn't justify the price. The 15" rMBP also launched at a fairly steep price, but at least it came with the hardware to justify the extra cost over the cMBP. The 13" should have followed a similar pattern. Apple should've either given it a serious upgrade performance wise or priced it more competitively.
Why does it matter that the 13" MBP never had a dGPU? The retina displays are new territory. Apple freed up a lot of room by removing the optical drive, removing the ram sockets, and using a proprietary SSD. This should've allowed them to put in a dGPU. It doesn't necessarily have to have the same CPU/GPU as the 15"... but it should have something better than what it does right now. A dual core i7 with an actual GT 650 (rather than what's effectively a GTX 660 in its big brother) would have made sense as a compromise.