I'd second (third?) what LoneStranger and OregonMouse say. I use one of those 6-liter bags when I need to cache water, or transport water to a trailhead or a car camp, but I'm hesitant to use them on the trail - particularly to trust all my water to them - because of that long zip-loc closure on the top. It just seems like user error, or a bit of dirt caught in the track, could lead to a zipper failure and you're left wondering what's dripping onto your legs and why your sleeping bag is soaked.

Also, like they say, 3 of the 2-liter bags give you insurance in case one bag should develop a hole. Multiple bags also give you the option of moving some of the water load around in your pack to help balance it as you eat food or clothing goes in and out of the pack during the day.