A rain cover is a great middle-ground option if you ski in variable conditions and want full waterproof protection without paying for a fully sealed pack. The cover stows in a pocket and deploys over the entire backpack when the weather turns, keeping your layers and gear dry in heavy wet snow or rain.
The trade-offs are worth knowing: a deployed cover adds a bit of weight and can catch wind on chairlifts or ridge hikes. It's also something you can lose if it tears off or you forget to repack it. For most resort skiers in dry climates, a DWR-coated pack is perfectly fine. But if you regularly deal with coastal snow, spring slush, or unpredictable storms, the rain cover gives you reliable insurance.
One smart habit regardless: stash your phone and electronics in a small dry bag inside the pack. No water resistance system is foolproof, and a dry bag weighs almost nothing.