IMHO, the sleeping bag is the most important piece of insulation and needs to be kept dry at all costs. After taking a tumble during a dicey stream ford some years back, I found that my sleeping bag and extra insulating clothing kept in a dry bag were still absolutely dry, while everything else was exposed to the water (several inches deep in the bottom of my pack). Fortunately anything else vulnerable (like dry food) was in plastic bags.

Since the weather was cold and rainy, I would have been in serious straits had my insulation gotten wet!

I do see the advantages of a pack liner, but would switch only if the liner could be sealed against possible immersion and were not slippery. I tried a plastic liner one year, but it turned every morning's packing up into a fight, repeatedly stuffing small items down into my pack only to have them pop right out at me again. After that frustrating trip, I found that eliminating the slippery pack liner saved me at least 15 minutes and lots of frustration each morning.

Your vital insulation needs not so much defense against heavy rain (although that can be a problem) but the dangers of immersion. I found testing waterproof containers before each trip (in the bathtub) is an excellent idea.