I do carry a lightweight (1 oz.) pack cover to keep the outside of my pack dry. It's my pillow at night, and I sure don't want a soggy pack exterior next to my sleeping bag! The pack cover, though, is worthless if you fall in the creek!

I use dry bags for my insulation items (one for the sleeping bag and the other for my insulating clothing). The two combined weigh about the same as a pack liner. I gave up on the trash compactor bag pack liner after getting throughly frustrated having small things shoved down in my pack pop right back out at me thanks to the slippery plastic. I swear that eliminating the pack liner saved me at least 5 minutes packing up each morning!

YMMV, of course!