As you rightly pointed out, there are games that take advantage of >4GB GPU memory, but those are still few and far between and most importantly, they are generally more texture-heavy than processing-heavy (open-world games like Far Cry, Elite, GTA, Skyrim with extra texture packs). If those games are your primary use-case, then yes Hawaii + 8GB is going to be better for you than Hawaii + 4GB.
But in my opinion, games that are "imbalanced" in this way (heavy on textures, light on GPU) aren't (or at least, shouldn't be) the future of gaming. Unfortunately, due to certain realities (consoles and the age of DirectX 11), game engines just haven't been able to properly balance great textures with great particle effects, highly detailed models, etc. DirectX 12 will (I fervently hope) end this stagnation and give us a new generation of game engines that need the horsepower that Pascal has and Hawaii doesn't. I'm talking the graphical leaps we saw going from Unreal Engine 1 to 2 to 3, not sightly more detailed tessellation.
In all honesty, Fiji was the right time to make 8GB the new normal because Fiji was the first arch that truly had the horsepower to drive 4K; unfortunately just as Hawaii + 8GB is mostly hamstrung by Hawaii, Fiji + 4GB was mostly hamstrung by the 4GB limit. AMD mitigated that somewhat with the system memory cache, but in all honesty Fiji with 8GB HBM (and maybe a few extra ROPs) would've blown GTX 980 Ti out of the water, and I'll be very interested to see how Vega (which should essentially be Fiji+ coupled to 8GB+ memory) will perform in regards to validating that theory. nVIDIA just happens to have not fumbled the ball in regards to coupling 8GB memory to a GPU that can actually use it well, and probably they don't really deserve that credit, just as AMD doesn't really deserve getting pissed on for trying to push 8GB ahead of its time. But such is the way the chips have fallen.
I purposefully ignored Crossfire for the simple reason that it's mostly irrelevant, because the vast majority of people don't have CF setups; they have single GPUs. That means developers are going to optimise for single GPUs, and that means there are really no games that were coded to exploit the horsepower of 2x Hawaii. But Pascal has that amount of horsepower and 8GB memory on a single card, which means it's going to become the new optimisation target - and also means, ironically, that 8GB Hawaiis in CF might be a better investment now than they were when first released...