EDIT: Noticed that several posted the humidity factor of down and loss of thermal efficiency. Of course synthetic also loses thermal efficiency. And add in the fact that when your body temperature heats the bag up, said thermal efficiency comes back as the water vapor leaves. Took too long posting this as I went away and came back and hit post and then saw several other very good posts above this.
If you were referring to my posts as to why Down is ok in rain, I think you might have missed a very key point. Not that I blatantly typed said point, rather a point that is not obvious at a quick glance.
In rain, as you typed, BBags get humid and trap moisture. You can't get away from that fact. To dry them out you have to use your body heat every night. If you expect 30 degrees and take a 30 degree down bag you won't have as much insulation as you thought due to said humidity build up and the ability to stay warm is hard in rainy conditions on top of that as what you will be wearing will also be wet unless you take 2 sets of underclothes, or baselayer clothing, which everyone here in the ol' PNW does.
If you have a Barely there sleep system for said temperatures and then add rain and humidity, drying said down sleep system out is difficult to do. Why? Because there is less "dry" or even semi-dry insulation to trap said heat and warm up the entire bag and you. Certain sections of said BBAG will let more heat out. Since your body only produces "x" Watts of heat a BBAG with less available insulation will not heat up and will therefore not dry the sleeping bag out. It is also not a linear relationship either. If you really want to know, open a Heat Transfer book.
So, one needs a sleep system that surpasses the temperature point to take into effect high humidity and rain. Generally around 30F-40F is the high humidity point, or worst conditions. I have personally always used down bags in lots and Lots and LOTS of rain. Of course I need said extra temperature rating along with my clothes for much colder temps at higher elevations and this gets me through the hypothermia 30-40F points. Heck, I have routinely camped outside in light rain without a tarp till we finally gave up and went home, but said bag also had Goretex face fabric, but it most certainly was not seam taped, this does not count as it was hardly backpacking "light". Though I was camping for a weekend with no shelter, so I suppose I "saved" weight. Would have been happier with a shelter and a lighter bag though as playing cards in the rain, truly sucks. Add even a tarp and it would have been enjoyable even with only 24" height from stacked rocks and ice axes.
Now, I have also been stupid and tried sleeping in just my down jacket and a bivy bag. Sure enough, down jacket lost its insulation power as it got humid and damp and then I got really really really cold. But take same humidity and dampness that was in my jacket and insert it into a down bag good for "20F" while its pouring rain out and even though said bag is heavy with humidity it has the ability to trap said body heat I generated and dry itself out as it doesn't have a giant seam at the waist.
PS. I used to work outside in the rain day in and day out at a greenhouse and nursery. Why I know for a fact that several name brand Goretex shoes/boots are not waterproof. Rather cheap work boots with good ol' wax is light years better. I got to the point where unless it was raining hard, I would just wear fleece 200 and after it stopped raining between my body heat from working hard and how little water the fleece retained, it would be dry in 2-3 hours. No DWR on said fleece. No shaking out of said fleece. Now if it was raining hard, I would strip down to shorts and a T-Shirt and yellow bellied rubber waterproof pants and waterproof top. I would be drenched in sweat if I had to run and work hard even in 40F pouring rain. But, I would have dry clothes later.
Rain, if you move in it, take as many clothes off as possible and either just get wet, or move slow and don't sweat.
Snow, just wear as few clothes as possible and don't sweat.
In either case you gotta watch for hypothermia. Why its better to be cold and know it, keeping dry clothes in your pack than to wear said clothes getting them wet and have nothing to put on when the temperature drops further yet.
I have used synthetic bags as well. When I had no money, I would hike with a coleman polyester bag. They would get wet just as readily as a down bag and they were just as hopless when it came to insulating power. I also never saw them drying out much faster than down. The difference is that Down losses its insulating power faster for the same amount of wetted bag as down feathers clump up when they get wet.
I have personally not used modern synthetic sleeping bags. I have rubbed "elbows" with them. Though I remember one friend buying one and immedietely sending it back as it was not only over a pound heavier, but also far far bulkier and hard to pack.
I would admit that if you are going to be using a bivy bag where one is far more likely to rub against condensation or no bivy bag at all I would grab a synthetic bag as they absorb less water than down, thus your body heat has to evaporate less water.
BUT: I don't know about you, but I don't plan on hiking in the rain for weeks on end. I know, some poor folks hiking the last stages of the PCT in Sept/Oct here in WA can get smashed. Read several reports like that.
PS. Want dry feet in the rain? Get real leather boots that have been sealed with wax. Need such contraptions for snow anyways. I know anathema to the light weight tennis shoe only crowd. Goretex lined shoes/boots are not waterproof. Water will seep in at the bottom where its stitched into the sole to start with and second of all, if you aren't wearing VB socks in the rain, then your feet continue to sweat, soaking said socks/shoes even more. In non rain weather on leather shoes, your feet actually evaporate quite a bit of water vapor out through said leather.
For an excellent write up on Vapor Barrier and why you NEED it. Look at warmlite.com and click vapor barrier. It is long, but if you Plough through it to the end, you will learn several useful things. I know I did.
PPS. To the OP. I have done plenty of snowshoeing and winter climbing in the PNW. If you limit yourself to low valleys then the humidity is far higher as the temperatures will be warmer and closer to 32F. The higher you get, the better it is. As winter ends and spring begins, you can get very high humidity at high elevations as well as low. Usually this is not the case in the winter.
Stirs the pot and checks to see if the mud is properly distributed for maximum murkiness. Yup, murkier.