It's absolutely essential. You have to keep drinking water, all day long, and by far the easiest way to do this is with a bladder. Get something at least 2 litres. You do not want to run out up a long hot climb.

Most in my group also carried another bottle as well, which was used for sterilisation and mixing electrolytes into. Some had nalgene while I just used a 1.25l PET (i.e. softdrink) bottle. Fill this and the into the main bladder when it's sterilised, which is about 30mins or so if you're using chemical treatment.

Regarding sterilisation, we had a mixture of iodine tables, chlorine tablets and steripens (UV treatment). Nobody suffered from any bugs so all were successful. Take your porter/guide's advice about where to collect water from which is probably the main reason we avoided any nasties.