If it has Wacom level of sensitivity/functionality then that is a different story, then I would have to re-evaluate it on that premise. I am not sure it has that though.
The dgpu will likely make it perform faster than the 13" and 15" igpu macbook pros (substantially so in the case of the 13") but it will lag in productivity against the 15" quad core and probably just match the 13" broadwell MBP. It is an improvement and I would agree that it should increase the price, but not by 800-1000$. The base price of that dgpu version is $100 short of the Xeon-based Mac Pro. That is excessive.
My rhetoric may be a bit overblown but my qualitative assessment of this product remains valid. I don't see why you wouldn't just go Surface Pro 4. To game?
I totally agree that the sensitivity of the stylus and digitizer is pretty critical. If it sucks it isn't worth it IMHO.
As far as needing the dGPU goes, Photoshop can benefit from GPU acceleration for several features such as liquify, smart sharpen, perspective warp, and some of the newer blur filters. When you are trying to apply a perspective warp to a 5 exposure HDR merged RAW file from a full frame 20 megapixel DSLR, you are gonna see some significant lag. I do a lot of HDR imagery as well as a lot of stitched panorama HDRs. Those tend to drag even my desktop PC to its knees when I am applying some of the more complicated filters to them.
I also do some video (mostly of the time-lapse sort) and that is also a laptop killer. I believe Premier can also benefit from GPU acceleration for rendering as well. I remember once trying to process some time-lapse videos on my MBA while I was traveling, and it was an overnight process. My poor MBA fired up its fan and chugged on those for numerous hours.
Admittedly it is a very expensive machine. But if that stylus is good it will be worth the extra cost over a dGPU macbook pro for me.