I use stuff sacks/dry bags for my important gear. I certainly use a compression sack for my sleeping bag that also serves as a dry bag. I do not like liners because they trap water inside the bag. Keep in mind that your issues with water will not come just from rain getting into your bag from the outside. At some point you will use your gear in the rain (tent for example), ad then you will put it back in the backpack. All that water will get inside. A liner will keep it inside the bag, and if your gear is not in independent stuff sacks inside, there may be problems.