Only just spotted this thread... o
On the PCT I walked almost 2000 miles before getting caught out by a stuffsack that wasn't waterproof (God bless California sunshine). That was the day I'd been hiking with an injured French Canadian through torrential rain (Sisters Wilderness area, Oregon) and we took out our down bags to find they were heavy, sodden, clumped up lumps.
A night in those and we'd've been hypothermic, without doubt.
Fortunately, we discovered this mishap while stood at the bar of a very remote lakeside resort that had opened specially for us when the owner saw us struggle in out of the rain. He gave us a cabin at a very special rate and saved us from a great deal of misery.
Trail magic is real!
Since then I've always used a proper sealed drybag (various brands) for my down bag. As my sleeping bag of preference is one or another PHD Minim (I've got a couple of slightly different models) it packs small enough without any serious compression so volume isn't an issue I need worry about.
Watch the seams on those dry bags though. I tested an Ortlieb one several months back and one of the sealed seams burst open. Still haven't heard back from them as to why, but it hasn't happened with the replacement I was sent.