Thats coz of the CPU bottleneck. If we had a CPU good enough like the Upcoming FX-59, Dual Core 2.6-2.8 Ghz system, whcih MAY be able to reduce the bottleneck to nil.
Two 7800 GTX's with a proccessor which holds its own, will own. Only then will it not be a waste.
Right now, 2 * 7800 or 6800 even, is a Waste!