Re: PCI express over a cable
You're answering the wrong question. What is its worthiness as a display interface?
I can see it saving a bit of hardware and connector real-estate in compact machines, especially ones with a graphics core in the CPU. At that point, it becomes useful to have it even on discrete graphics cards, for the sake of having a common interconnect on everything, even if it doesn't necessarily offer any technical benefits to things like discrete graphics cards.