Maybe you can plug it in. But then you'd be tethered to the computer.

However, Mac might not have low enough latency / high enough bandwidth for best experience. My thinking is that it will be an ultra-optimized highly integrated device designed to do just AR/VR, and that's also why it can beat all competitors which interface with standard PCs. I have a very high-end VR headset and GPU, and it just isn't good enough. Apple has to make some miracles here.