[citation][nom]pinkfloydminnesota[/nom]How on earth can you give an honorable mention to the 5970 when it's about equal to the Xfired 5850s and 20% more $$? Do you really believe single slot motherboards exist on any systems with $600 invested in graphics?[/citation]
The performance isn't equal, and with very high end parts, the last few percent do tend to cost a lot more. But more to the point, if you have your 5850s cross-fired, that's as far as you go. If you have a one-slot solution, you still have possible expansion.
Also consider the brain-damaged lga1156 platform. Many of the solutions use PCIe lanes from the processor for the USB 3.0. Even without that you'd be forced to use them with 8-lanes instead of 16, but with it you'd see some degradation either in USB or your video card.
Also, some motherboards have PCIe 16 slots kind of close to each other, and the cooling can be an issue. One card allows for better airflow.
So, there are good reasons for wanting the one card solution, some of which depend on the existing platform and configuration.