>>64164540
Never really used one but they seem like too much weight and bulk to have in one spot. I've posted my setup here before (picrel) and two balanced 1qts on either side gets a bit cumbersome at times. Who'd want more?
What I do for extra capacity is I keep a 2 qt bladder I got for like 10 bucks in my backpack's camel pouch. Only had to use it twice and when it's empty the thing takes up basically no weight+volume