In this case one would do so to let the card compete on an even playing field. If you are going to compare two different cards it shouldn't be against one that has been downclocked significantly for an arbitrary power spec. That and as awseome as the 5970s are running 2560res on 3 screens in intense gaming may require a smallish bump to maintain FPS.
Who is underclocking in order to meet an arbitrary power spec? Nvidia lowered Fermi's clock in order to stay within the PCI Express standard thermal limits, which are rather not arbitrary (since they're what a case can be expected to handle without damaging other cards). Even then, they still have special cases with special video card cooling for these cards.
If they really wanted to crank up the speed they could, but it'd put out so much heat that it'd break the standard and they couldn't call it a PCI Express card anymore (not to mention no OEM would go anywhere near it).
Nvidia hasn't really done much in a while except rebrand existing cards with new numbers and put out drivers that fry cards. At the moment, ATI is dominant. That usually doesn't last, by this time next year Nvidia should have something competitive again.