I have a 3L "nalgene canteen" that I use for a lot of the purposes posted here. It weighs 3oz. One of the biggest benefits I have found of using a separate container for treating (I use chemicals) is that the dirt all settles to the bottom when it sits for the 20-30 minutes to treat. I can then pour off the top into my camelback, dump the grit/silt, and not be stuck drinking dirt for the last 5 sips of water.
It's definitely not "needed" but it is nice.