It's stipulated that AMD will be reducing the size of the chips with Navi. If, instead of relying on large chips to perform well, they rely on the "scalability" of multiple smaller GPU's on the same die, Crossfire might be the way to go. This is just a stipulation from AdoredTV, but it makes sense. He believes AMD has a "Master Plan" that involves shrinking chip sizes—to improve yields and reduce costs—and putting two smaller chips on the same interposer. With the new API's out at the moment, and the next generation of consoles likely to powered by AMD again, if AMD can persuade Sony, Microsoft and Nintendo to go with a dual-GPU APU for 980ti-like performance, without the huge costs and efficiency problems of that, the game developers will be forced to develop their games around DX12 and Vulkan that better support multi GPU's. This means it won't be down to drivers from AMD or nVidia; it'll be the game developers themselves that make Crossfire and SLI more attractive.
That's the theory. Also, VR loves multi GPU's, and as NeverBackDown is saying, it's here to stay and is a big potential market. If VR developers want multi GPU's, AMD and nVidia—and also the game developers—might focus on it more.
It ties in with Moore's Law as well.