I respectfully disagree with this perspective. I enjoyed Nibali's descending at the Giro, but felt that it was ineffectual tactically. Only once might it have made sense, and that was when he caught up after he had been dropped - but it took so much energy, he was quickly dropped again on the next ascent. The time he "attacked" out front was fun, but literally laughable tactically. If you could have drawn a cartoon bubble over the group when he took off, it looked like they were saying, WTF? He ended up isolated on the flat and soon decided to soft pedal until the group caught up. All this was not his fault, it was just that there were no opportunities for him to make gains with that skill.
Contador, on the other hand, appeared to be a way better descender than many people gave him credit for. But rare are the moments when it makes sense for him tactically, considering the risk/reward of his position in a series. Certainly he had no incentive to follow Nibali at the Giro - actually it made no sense for anyone to do so.
At the Tour however, as soon as it made sense tactically he showed himself to be a very capable descender in proportion to his other skills.
Lombardia with Gilbert, Sanchez, and Contador ... and Nibali and the others ... wow!