Totally agree. It adds cost, size, weight, power consumption and technical issues for all users, although very few will benefit from it. Iris 580 is more than enough for the vast majority of laptop users.
If you want a cutting edge gaming setup then you shouldn't be using a laptop in the first place.
p.s. the Iris 580 has more than 6 times the GFLOPS of my current MBP's dGPU. I won't be complaining...
I agree about cutting edge gaming but there are people who just want one computer that can play casual games as well as perform the other tasks expected of it. I don't think that's unreasonable for a £2000 machine. Given Apple's Mac sales are declining significantly it would be an odd decision to force some of their potential market to buy someone else's product because of their rabid devotion to thinness and battery life.
p.s. The Iris 580 has about the power of the current AMD dGPU in the MBP. This is lower than the recommended system requirement to play the upcoming expansion of World of Warcraft, a game that is pretty easy on the GPU. If you are happy with that, good for you, but in the absence of an eGPU option I would move away from Apple and their sales will be a little bit worse.
FWIW I think they will put the upcoming Polaris chip in the MBP when it is available. Seem like a perfect fit given the power per Watt gains.