Do I actually need a rain cover, or is a pack liner better?

Specialist community threads on r/Ultralight, r/AppalachianTrail, and r/backpacking are split. Many thru-hikers favor a trash-compactor bag or dry sacks inside the pack because covers can flap, wet out, and let water pool at the bottom. A rain cover still has fans for shorter trips, commuters, and anyone who wants the pack fabric itself to stay clean and unsaturated. The most common compromise reviewers describe is using both: a liner inside, a cover outside.