2 litres in a hydration bladder weigh 2kg, and I certainly wouldn't think of carrying that sort of extra dead weight. Except in really hot weather, I have half a litre to hand, and if I think it necessary, half a litre, to be used in desperation, buried in my rucksack, and I will top up when I can. Anyone who becomes medically dehydrated in this country over the course of a day with a litre plus available has to be doing some very strenuous exercise in untypically torrid conditions. More often than not, except in summer, I'll walk for five hours before I even think of taking a drink. Until someone convinces me otherwise, I'll continue seeing hydration bladders as a marketing initiative/fashion accessory.