Ahhhh; the age-old question. The answer hinges on one word: acceleration. Together, you and your bicycle are one big mass, and it takes a lot more energy to get that mass moving than maintaining a steady speed. As long as you are rolling along at a steady speed, wheel weight is not any more significant than the mass of your bike or body. The crow in the omelet, however, is that the top of your wheels travel at twice the speed of the bicycle ? So the energy it takes to accelerate the wheels is compounded.
Lightweight wheels are built with weight savings in mind because when a grade becomes steep, the bicycle slows and then accelerates slightly between each pedal stroke. The extra energy that this pulsing motion eats up can be considerable, so reducing the mass of the wheels-especially the rims and tires can save a lot more energy than removing considerably more mass from the bicycle or rider.
As a side note: watch the climbing stages of the next Tour de France and notice how smoothly the pros pedal. This is no accident. If you can learn to climb with a smooth, circular cadence, you will eliminate wasted acceleration caused by pulsing, and can use that lost energy to reach the summit.