I like to carry a combination of 1L aquafina type bottles (wide mouth) and Platypus bladders. if i need 2L, i'll just carry two 1L bottles. if i need 3 or more liters, i'll carry a bottle or two along with a 2L platypus.
i have a 3L big zip and it works well, but i don't use the zip part to fill the bag. i only use it to clean and dry it. though i have never had it fail, i don't like to open and close it all the time; just fill/pour with the regular opening. i only used this when i need 6L or so on the PCT here and there. now i would just use a 2 or 3L regular platy (non zip).
a platypus is not that easy to fill from a stream sometimes, but that's why you have a 1L bottle too. just fill the bottle, pour in the platypus, and repeat as necessary (2-3L). easy.
re: overcarrying. yes, water is VERY heavy. one of the best thing you can do to reduce packweight is lighten the containers and carry just enough water as needed. this requires more planning, cameling up, knowing your abilities (and reaction to mild dehydration!). all the benefits of pack weight reduction one makes can be countered by carrying several extra, unneeded liters of water!