Actually...you do. Or rather, many of us do. You've apparently not done much with gaming. Even a very old game like World of Warcraft practically crawls running side-by-side next to a non-Retina 15" because of all that extra rendering. It's even worse with newer games.
Well, I used to play games but not that much anymore.
So I do know that world of warcraft is 3D, you are talking about 3D processing now, not just pixels anymore. 3D calculation you do need GPU to be fast.
I understand that some people may want a dGPU, I am ok with it. We don't ever refuse a fast GPU do we? It's just that the iGPU is definitely fast enough to deal with the retina monitor itself.