Two trains of thought regarding liners, semi firm ones (some are even insulated) allow you stuff and fill bag easily with your gear...downside, even collapsed they are bulky or if you constantly use the zipper, they fail.
Now, if you are in a hurry soft inner liners are not as convenient, but collapse easily, are less expensive and zipper use is limited when you are using them for stuffing to the max. Things are never easy, but do take your time and choose wisely...