That reminds me of my other realisation about descending quickly (which was borne out at Dunnerdale):
It's tiring!
So if you want to do well at long descents, you need to be fit enough or fresh enough to have the energy to push. If you're still completely shot after the climb, you won't be able to descend quickly. It's a balancing act - where do you want to gain places? Where do your strengths lie? And where is there the most time to be gained?