For gaming it only uses one GPU as we know. It's still a more than capable card if you want to run even with all bells and whistles in most games on at 1080p. It'd be a lot faster under Xfire but as you said that's not happening.
Then again, this is not a gaming computer. In fact, people would be acting pretty unreasonable and contrary to known fact to purchase this expensive beast for gaming when there are much cheaper alternatives for similar quality (heck even a PS4 competes on gaming and that's 10x + cheaper). If they buy it for gaming alone, it's purely an emotional not empirical purchase.
This is a creative professional's computer. The 2nd GPU is used currently purely for creative software, where it allows people like myself to render & export out much faster than was possible before on, say, an iMac. It allows me to run Resolve with 2x the GPU power and get many more nodes running in real time or close to. It allows us to edit 4K fluidly in real time. You need that 2nd GPU to get that done. And it's been proven the AMD chips run OpenGL much faster than Nvidia, though that will change as drivers improve.
Even Dual D500's are faster than a single 290x card for getting work done. The 6 core base model is super fast for getting work done too.
Anyone can build a more than capable souped-up PC gaming machine for a fraction of the cost of a Mac Pro. If people want to game, and I mean core and not casual gamers, they should get one of those, or buy a PS4/XB1. The Mac Pro is extremely fast... at getting work done.
If you're a once in a while gamer and want good 1080P gaming as a bonus when you have some down time for work, (like me) then consider it a bonus... not the reason to buy the machine.