FWIW, using a (thick) plastic bag serves the added purpose of keeping everything dry. Many of us did a variation when we mobilized to Afghanistan, placed thick heavy lawn/garbage bags INSIDE of our 3-4 required duffle bags. Worked quite well even when the bags were left outside uncovered in the rain for hours at a time.

I’m not a back-packer, don’t some people use plastic bags to keep their packs dry when they're caught outside in the rain, similar to a poncho placed over a backpack? 1 or 2 shouldn’t be significant extra weight and can be folded/rolled into a small volume.