the Intel integrated graphics can make use of up to 64GB of system RAM. so...
One thing is the hardware, and another the OS support. I'm not really sure if MacOS can currently dedicate 64GB of system RAM to the iGPU (in past recent OS X versions it used to have a rather low OS-imposed limit, even if the iGPU hardware was able to lock more system RAM).
Anyway, you make a really good point, and you really explain why I consider the Retina MacBook a much better option than the new MBP: If you need to deal with visualization of huge datasets (like in my case), the new MacBook Pro makes compromises I cannot take: Dedicated GPU with too low VRAM size, and system RAM limited to 16GB. This means the new MBP can not replace my desktop without a serious performance decrease (which, BTW, isn't an Apple Mac anymore, just because Apple didn't -and still hasn't- have a desktop Mac with a current powerful GPU anymore).
I've a late-2010 MacBook Air. I cannot use it for huge visualization sets, but I can use it for the rest of things: It runs AutoCAD very smoothly (even if the guy at the Apple shop insisted me to buy a MBP at the moment, arguing the MBA wasn't enough for AutoCAD: I insisted I knew what I was talking about, and that the MBA was perfectly suitable for AutoCAD). It also runs smoothly Mathematica, GIMP, LLVM compilers, Office, and all the stuff I use in my everyday work. The only performance problem comes with OpenGL and OpenCL.
If I buy the new MBP, I'll get a laptop which weighs almost twice my MBA, while not being able to replace my desktop for visualization. The cost of the new MBP is roughly equivalent to what I payed for my desktop.
But then you look at the Retina MacBook. I didn't check, but I guess its performance won't be lower than my late-2010 MBA (and, if it's lower, I suppose the likely future update to kaby lake will get to comparable performance levels).
With the Retina MacBook, I'd get a laptop which is even lighter than my MBA, can do the same things I use my MBA for, is aesthetically very beautiful, and has no fans (which I consider a very important point: if you cannot replace a desktop, then make it absolutely quiet, no fans please).
The new MBP is a mid-road compromise: It's heavier than the MBA and the RMB, but the hardware isn't really professional-level (performance-wise). I travel with my laptop a lot. If the new MBP had more system RAM and/or a more powerful GPU, I'd happily chose the MBP even if it's heavier than the RMB. But with such limitations, it doesn't justify losing the lightness of the RMB, at least in my case.