An exterior cover can’t keep the whole pack dry in heavy rain. Water can run down your back and soak into the pack that way. Things that really need to stay dry, need a sealed bag.

But you don’t need a liner for the whole pack, either. Cookware and stoves can get wet, and the tent is probably wet anyway if it is raining.

Typically only the food, sleeping bag, and clothes really need to stay dry, so you can just seal those each in their own bag. I kept those things dry for a month in the Pacific Northwest using just regular kitchen trash bags.

Using dry bags inside the pack means you:

  * Don't need to stop/remember to put the pack cover on.
  * Can use them to organise different things.  e.g. separate clothes from sleep kit.
  * Can use them to roll/squash the air out of clothes/bedding to take up less space.
  * If you need to remove things from the pack to look for items lower down they'll stay dry in their separate bags.

The trade off is a little more weight but it's generally < 100g-ish compared to a pack cover.

Depends on your use cases :)

For me, a rain cover is a hindrance when I need to retrieve things from my pack, I have to remove it entirely, whereas a pack liner, I just need to unfurl the top of it.

Secondly, as there are very few bridges in the New Zealand back country, we frequently have to cross the channels of braided rivers. A rain cover keeps out water from above, not so much from below.

And as I mentioned in a previous comment, pack liners can, in an emergency, function as a flotation device or emergency shelter.

I have a backpack (shout out to the REI trail 25, I've put it through an absolute beating over the last 4-5 years and it still looks and works great) I use for around the town kind of things, day hikes, trail work, etc that has a rain cover. They're alright, but it's a bit of a hassle to get it out and put it on the pack, and vice versa.

It's nice if I'm biking home with the pack on and it starts raining, but I don't bother if it's going to be a short light rain.

I use both. The stuff inside the liner bag is stuff that has to remain dry (extra clothes, sleeping bag, stove and matches/lighter, etc). Anything outside the liner but still under the cover is stuff I'd prefer remains dry-ish, but can get wet. I made my cover out of Silnylon; it weighs very little compared to the added peace of mind