Some of us use multiple water containers with the Steripen because we don't want to carry "heavy" Nalgene wide mouth bottles. Platy bottles or commercial water bottles are so much lighter, but have tiny mouths that the Steripen won't fit into. So I carry a Nalgene "canteen" with the lid cut off, making it just a one liter heavy plastic bag. I use that to dip water out of the sources and treat with the Steripen, then pour it into my Platys. Still much lighter than the Nalgene bottle.
On another note, I picked up a Sawyer filter. Lighter (dry) even than the Steripen, filters quickly, 0.10 micron filter, very long life. I've used the Sawyer filter before and liked the results as a gravity filter, but it was a lot of fussing to get everything set up, and then took awhile to do the job. This seems very quick, no fuss, quicker even than the Steripen. So far I've only tried it with tap water, but it'll go on my next hike!