AMD cards with 8GB of VRAM were all using Hawaii, which doesn't have enough horsepower to drive the higher resolutions/VR that would actually use that extra VRAM. Pascal does, hence why the 8GB is actually useful.
No, the reasoning was that nothing really could take advantage of 8GB, not that it can't use it. I've seen some games use more than 4GB on my 390 and it runs just fine but, there are other GPUs that use upto 3GB or 4GB that still run just as well. W1zz has even stated that most games, even at 4k don't tend to need more than 4GB yet but, there are games that will use it if it's there. The problem with saying that Hawaii doesn't have the ability to drive the VRAM because it depends on the workload. Generally speaking, it isn't compute that takes up a lot of VRAM, it's textures, and Hawaii has quite a large number of TMUs and has some pretty significant texturing capability. I was able to play Farcry 4 in surround with AA off without too much problem and there were occasions where I used just over 4GB. Same deal with Elite Dangerous, in some situations (with the 64-bit client,) more than 4GB of VRAM could be used, in fact I saw usages almost as high at 5GB but, that isn't to say the GPU needs all of it at once or that the 390 can't handle it.
Either way, I still want to know the reasoning behind 8GB not being a con. It's either because nVidia is doing it which now makes it "normal," or games have evolved enough where 8GB can actually provide some level of tangible benefits.
Also, even if I were to play devil's advocate and say that the 390 can't handle 8GB worth of whatever gets put in there, I would argue that wouldn't be the case in CFX as I can attest to personal experience that more memory for multi-GPU setups is generally a good thing having come from CFX 6870s.
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