Hobbes,
Everyone's different in their water requirements. I drank 6L on my way up the mountain, and one on the way down (which probably should have been 2-3L). I carried 3-4L for the trip from Portal to Trail Camp, filtered/filled there, and re-loaded for the rest of the summit. Had the option to refill again when I got back down before the final push back down.
Admittedly, we had two filters within a group of five people, so it was easier to carry a bit more than have to make a 30 minute stop to filter enough water for us all.
Plenty of people hike with less, but I hear lots of stories of people on the back side of Whitney pushing for the summit who ran out of water. I'd never want to be in the place. I always carry a little extra water with me everywhere I go, and never have to worry about rationing it out on my hikes.
All personal at the end of the day, but I never mind carrying a bit of extra water.
