Okay, assume you will get wet under your rain shell. You stop hiking and there you stand, it is 45F and you are tired and wet and starting to get chilled. Assuming that it isn't pouring, you get a dry layer out of your pack.... but UL principles lean to not carrying spares, or mid layers that might be base-layer substitutes. Oops! Many UL gear lists would have something like a Thermawrap or a light down sweater for insulation, with no spare base layer. I don't like that idea.
This is one of those scenarios where I think light fleece or other stretchy synthetic mid-layers are great. Power Stretch is my favorite, but there are tons of options, from basic 100w fleece to R1, Capilene 2/3/4 and so on. If I have stopped and find my base layer beyond redemption and I am cold, peeling and using my mid-layer for a long stop or camp would be my plan. The base layer might dry if I can get it under cover and a breeze, or a little warmth off a fire if possible. If it isn't soaked it will dry from my body heat-- if I don't get too cold trying to do that. Having a second base layer might be in the works too.
1. IMO there are two ways to get somewhat less damp at the end of the day ... spare base layers ... or using body heat and hawt nalgene to push the moisture out (which requires extra fuel and over calculating yr insulation) ... or both ... neither is "UL" ... base layers need to be as thin and form fitting as possible to facilitate drying and wicking ...
2. fleece is the "best" for wet conditions despite all the hype about synthetic insulation ... it breathes well, has a fuzzy interior that "feels" less damp, and dries very quickly ... synthetic insulation has 2 nylon shells that need to dry out as well as the primaloft in the middle ... it also collapses more than fleece in truly wet conditions, it works for stops and in camp, but you should be careful not to get it truly soaked ... i wont even talk about down as we know that in 50% humidity 900 fill aint 900 fill, never mind 100% humidity
3. as mentioned its when yr stopped when you truly have to worry ... most people can generate enough heat or even too much with a base/light fleece/rainshell on the move ... but stop, and i dont care if yr warm wet or cold wet ... itll all become cold wet very quickly if you cant get under shelter and get more insulation on you ... or a fire
4. the trick IMO is wearing as little as you can on the move and the quickest drying things you can find ... that way theres less moisture in the system from the body, and less things to get soaked ... and dont stop for any real length of time till the end of the day ... if you do find yrself stopping often such as on more technical ground, have a light fleece underneath and control yr exertion levels
one thing i disagree with is using a windshirt here in light rain here .. unless you know it wont last very long, yr windshirt will get soaked and then the layers underneath ... much of the time in the PNW drizzle lasts for days on end ... better to put on a rain shell, wear little under it and use the zippers for temp control ... a good synth base will actually dry to just damp if yr moving as the body heat will push out some moisture with good ventilation
at the end of the day just remember that its all about the big 3 ... not the BPL ones ... but what they taught you in school ... fire (warmth including insulating clothing and hawt nalgenes), shelter (get the hell outta the rain), and food (including water, burn those calories) ...