ian,
I don't use either a pack cover or an internal liner and still stay dry in pretty awful weather.
Outside is a poncho because it protects your harness and keeps water from running down between your back and pack. Also jackets tend to leak below the shoulder straps.
Some items do not need to be protected from the rain like my kitchen and water system.
Some items like a hammock in water proof snake skins are adequately protected by the combination of the poncho and water proof container.
Most packs are made from a coated fabric that is highly water resistant.
Inside the pack are separate plastic or silnyon bags. And sometimes the contents of those bags are in bags. As an example Ramen noodles are protected by:
1. Poncho,
2. Pack cloth,
3. Food bag, and
4 Plastic bag.
Interior liners are very effective however. You can double line the pack with plastic bags and the contents will stay dry even if you swim the pack – short swims only.