I've been running dry sacks in my system for several years now, for those things that absolutely should not get wet. I run my sleeping bag, inflatable pad and insulating layers in a dry sack. I use an OR 55L lightweight durable dry sack for those things. It rolls or unrolls to adjust volume depending on the bulk of those items.
We are in a monsoon pattern here in western Virginia and I did a weekend trip this past. If it had not been for the roll top my sleeping gear would have absolutely ended up at least damp, if not wet. In a damp rainy situation I keep my bedding in the dry sack until a half hour before bedtime to allow it to loft up and put it away as soon I can after I'm up in the morning.
dry bags work very well and they've gotten very light- I have very small ones for things like first aid kits, fire kits, tp, etc up to larger ones for clothing and sleeping bags/quilts; some things like my pad, cookwear, rain gear, etc don't go in a bag- but anything that could be negatively effected by moisture goes into a dry bag of some sort
The Sea to Summit evac dry sacks with event on the base has kept my sleep system completely dry in a very wet pack and allows a guy to compress the air out out of the system making it quite compact.
I’ve been using the same USGI wet weather bag for 22 years now and I’ve never had anything get wet in my pack unless I put something wet in my pack. This includes putting air into it and tying up to swim with my pack.
The biggest reason I chose the Seek Outside Fortress 6,300 was because it is completely waterproof with its X-Pac material. No zippers, a roll top, and colossal volume that conveniently rolls down into a day pack. If I were headed out into horrific weather, I might have my sleeping bag in a dry bag for storage while it was still in the tent and only remove it minutes before using it as mentioned above.