>"This is why chemical treatment isn't very ultralight. The most ultralight way is to treat a liter of water with an instant method (filter, Steripen) and gulp down an entire liter on the spot."
Justin, I see your point and am now reconsidering my long-time use of chemical treatment. I much prefer iodine or Aqua Mira to squatting at a mosquito-infested water source and pumping. And I avoid the pump weight.
But since I end of carrying one liter (32 ounces) for 20-30 minutes, carrying a 3-5 ounce Steripen would be preferable if I'm drinking more than a liter every 3.5 hours. And then I could almost always forego the second water bottle because I could UV a liter, chug a liter, treat another liter and then hike a dry stretch.
Even in dry climates where you carry a few liters of water, you'd still save 20-30 minutes of waiting or 20-30 minutes of carrying an extra 1-2 liters if you lack instant treatment options.
I'm also treating less and less of the time and haven't encountered any problems. I judge each setting and always have iodine with me, but (1) I'm usually not hiking in the third world (and pull out all the stops when I do) and (2) if I get something, at least the medical care is close at hand and free.