[CmdrSprinkles]

Critical stuff you definitely double bag. But stuff like extra layers or even a sleeping mat are things that can get wet but probably shouldn't and adding the extra bulk of waterproofing them just isn't worth it (to me).

True waterproofing generally isn't worth it. But not having to worry about a light-moderate rain or getting too close to a river/waterfall (obviously not submerged ,but just getting sprayed) is very nice. For the heavier stuff, you should probably throw a tarp/poncho/whatever over even the waterproof bags anyway.

Although my personal feeling is that if I am caught out in a heavy enough rain that just wearing my softshell and relying on zippers isn't sufficient then I don't want to be hiking anyway and should be looking for shelter.

[avar]

I've done most of my hiking in Iceland, weather's very erratic, just because it's pouring doesn't mean you need to seek shelter, it's just rain.

I found that everyone I hiked with who tried to waterproof their backpack would end up with things soaked anyway. Frequently having to open the bag in the rain means the insides inevitably get soaked, it's much harder to even semi-waterproof a much larger area than having a waterproof duffel bag for the small stuff.

Gives you a lot of flexibility too, e.g. I've crossed rivers and then just dragged the backpack over to the other side via rope.