I saw the post from Patrick, just giving the hardware explanation of the whys and hows.
And over subscription will likely be a thing shortly, cause the PLX chips are there, they are just the multi-master/multi-root ones which carry a slight premium. For a x16 slot, you ideally want a 48 lane PLX chip like the 8750 configured in x16->8x4 mode. This will allow getting to roughly maximum bandwidth on a wide variety of workloads where as non-over subscription just ends up wasting PCIe lanes in many workloads. Also PLX doesn't make a 40 lane device which is what you would need for 6 connectors.