I use soft bag liners in my leather bags...but only when loading up for quick removal, when on the road... otherwise they are zipped and collapsed lying in the bottom. I prefer to just "toss" things in, not using the liners for speedier loading.
I use the liners only if I am packing stuff "formally" (folded items to keep clean and dry) that I want to pull out for the night at a hotel for instance. Hard ones would limit storage somewhat, get in the way on occasion, and I worry about wear and tear of the fasteners and what not.