When you think about it, many people with enough money to buy a 5870 or 480 usually want the best and will purchase a 2nd or 3rd for SLI/CF.
Plenty of people desiring the highest performing video solution (read: 30" or multi-monitor or 3d stereoscopic) may also be sensitive to input lag, notice microstutter or play titles which either have no multi-GPU profiles or exhibit poor (possibly negative) multi-GPU scaling.
So for them the only viable choice is the single fastest GPU. Which ATM is either the GTX480 or the 5870, overclocked as high as it'll go.
I agree there's a lot of value in seeing multi-GPU results in benchmarks. Most reviews already include the highest end hardware pairs though.
