The trouble with the larger water bladders such as the "Watersafe" is the prevention of bacteria and mold growth in the bladder after you use it.

Sure you could attempt to completely dry the bladder somehow or treat it by adding bleach or chlorine to the bag before emptying out as much water as possible. However once the small remaining amount of bleach or chlorine looses it's effectiveness after a few weeks, the confines of the bladder are the perfect bacteria and mold breeding environment which would be difficult, if not impossible to eradicate afterwards.