The performance level of the gpu's is not directly proportional to the level of bandwidth needed to communicate between the gpu's and the cpu(s), system ram and hdd's.
Nobody suggested otherwise.
So even if you have two cards that are "sharing the load", you are actually increasing the amount of traffic on the pcie lanes in folds, but it really depends on a number of variables such as the software and what information is being shared between these resources.
I think what you are trying to say is that SLI and CrossfireX are solutions that allow for workloads to me more evenly be distributed across processing platforms, but it doesn't necessarily increase the amount of traffic. Having a multi-GPU solution in place does not automatically increase the systems ability to keep said GPU's supplied with data.
This mostly effects how well a system is scalable when adding a 2nd, 3rd or even 4th video card.
J, what does this mean?
With SLI and xFire, the cross talk between gpu's are handled by the bridges obviously since pcie lanes can't handle the satuaration from high end cards. Although you can do SLI of low end cards without a bridge since you arne't pumping nearly the same amount of information.
When you run a multi-GPU solution the card mounted in the primary slot is the master card, and any secondary cards are considered slaves (much like data drives during the golden age of computing). When data is sent to the graphics processing array, the load hits the primary card first. Part of the package is offloaded to supplementary cards (how much depends on the solution being run, and how drivers are designed to dedicate the load). Data bridges are common on high end solutions to supplement available bandwidth on systems that run a 2x8 PCIe config. Today's standards probably don't dictate the need for a bridge solution when a 2x16 lane bus is available in concert with a chipset that could actually handle it (that is important too), but thankfully hardware and driver solutions are not designed with top of the line hardware in mind.
16 lanes is considerable bandwidth. For the foreseeable future I would imagine that most of the energy dedicated to improvement of the bus will be centered on speed and not width.