Interesting thread, great discussion :thumbup:.
I take a poncho-tarp on day hikes, and for backpacking I often am debating between the Silponcho (261g seam-sealed) or the DriDucks rain suit (159g for Jacket, 114g for pants - trail-use only, forget it for bushwacking or scrambling). Now, I will admit that most of my Silponcho use has been either as a sun-shade or wind break for chillin or lunch breaks, and on a few occasions as rain gear for brief spells without a ton of wind (forest). On a bone dry warm day, I will just put it up as a windbreak if overnighting...it's not the end of the world if the wind deviates because it's warm out anyway. The added plus for backpacking is that it gives me a porch or windbreak option for cooking out of the wind or a small common area if it's an easy drizzle and we just want to sit outside. Also can serve as a privacy screen.
My silponcho a couple of years ago, still in great shape today.
80*F days here, 0% rain prediction and balmy nights with mild wind predicted from direction tarp is pitched against. Could have easily done without the tarp and just cowboy camped it completely.
As sunshade. I do wish it was more opaque, but oh well, it does work well enough if that's all I have on me and not some more opaque dedicated tarp.
Would I want to use it as my dedicated rain shelter on a backpacking trip where I'd be expecting wind and rain? Most definitely not, much too small for my tastes, I want at least an 8'x10' for that. But, for day hikes, in conjunction with a waterproof AMK Bivy on a polycryo groundsheet or 5x7 BCUSA tarp, in moderate temps... I'd use it if it's all I had and was injured or caught out unexpectedly. Or if my main backpacking shelter failed catastrophically for some reason, the old "it's better than nothing" really holds true; one layer between my sleeping bag and the rain is better than no layer. I'd use the remains of my main shelter to extend coverage. For emergency survival use like on a dayhike, I would basically find the best natural wind-sheltered area I could find with good drainage, peg my poncho-tarp corners to the ground and A-frame it. Then I'd put my groundsheet under the area covered by the tarp, making sure it won't collect spindrift off the sides/ends. If I have insulation (my pack frame is a torso size Zlite CCF pad), plop that down under the tarp, then the AMK bivy goes on top with good coverage for my head and torso, and sacrificing my leg/feet end since the bivy should be waterproof anyway. I always carry a grocery bag for packing out trash on trails if I find any, so that would go over my feet for added foot-end protection. If I had time and mobility, I'd gather brush to door up the ends of the tarp to stop spindrift from blowing in (I'd have to stick my bagged legs through the lower end). Ideally, do like BTII said, use a tree to string up the head end nice and tight, then apply brush to the sides to fill in the gaps. Oh, and as for the hood, I just twist and tie a simple overhand knot, secure with hood cinch cord if needed.
This has always been the theory anyway, and I'm ashamed to admit I've never ACTUALLY tested it. So, although it won't be raining this weekend, I'll set it up and take some pics when I'm out hiking, maybe spit some water over it

, just as proof of principle. Now, if it was already raining, and it was getting dark and cold, it would be crappy situation setting up the shelter while getting wet. My only protection while doing that would be my windshirt, which can only handle so much before being flooded. The question of what do you do when you need to convert your rain gear into a shelter is a valid one, and I don't have a great answer other than be quick and efficient, unless you can pull off a trick like lmalterna mentioned.
Now, for use as dedicated rain gear when on the move day hiking or backpacking, no, I wouldn't prefer it. One is for wind, as mentioned, another is for not being able to see foot and leg placement when hiking wet and therefore slippery terrain. Plus, in a downpour, my lower legs just get soaked, then it wicks up and it all just ends up cold. Same goes for the arms, but that can be mitigated with my kit somewhat by using a windshirt under the poncho. And sure, I could get some silnylon chaps for lower leg protection, but it's one more thing... and I might as well just get a rain suit at that point. So, makes me a sail and catches on brush, can't see foot placement, and lack of full limb coverage = meh rain gear for actual hiking. I will say that I believe the poncho breathes better than rain jackets and pants, with the caveat that I haven't tried fancy stuff like eVent and the like yet. The couple of times I've used DriDucks, I can't recall being immediately disappointed by them, so it's a good sign for their breathability. But, venting is indeed easy with a Poncho. Most of the times I've hiked in the rain deliberately, I've used a rain jacket (Marmot precip, like the zip vents) and wished for rain pants. But for short rain bursts and general "just in case" day hiking or backpacking rain gear in clear weather, I call it good for the dual function as small tarp (more for wind and sun functions than rain shelter). It's compact for its weight as mentioned, can serve at least two non-critical purposes, and it's easier for me to think "sure grab the poncho-tarp" on ANY day hike, than to decide to grab both rain gear and another large-enough-for-shelter and therefore heavier tarp on a clear or ambiguous day for "just in case". I mean, if I had an 8x10 cuben tarp, it'd be a no brainer, take the cuben tarp and Driducks and call it good. Much the same like I'm leaning towards with backpacking these days: light main shelter and rain suit instead of poncho-tarp because needing the secondary tarp function isn't all that probable. And for day hiking, if I have the time, I'll just opt to wait out the burst under a tree in my poncho then move on when it lightens up. And if I really needed to move to get back to my vehicle, well, it may not be ideal to hike in, but it can be done. I'm willing to let my limbs get wet if I knew I'd be back to the car soon. The limb wetting thing is still something I'm trying to sort through, it seems some folks don't mind their lower legs getting wet. I just don't think it'd be very fun in sleet conditions.
Now, to address specifically the pack cover function of the poncho-tarp, I will say I am inclined to give it the nod there. For day hikes, I don't care; my pack can get wet because I use a liner anyway so everything is protected. But for backpacking, when it's possible I'll be out for several days in wet weather or when high humidity precludes speedy drying out of gear, I tend to believe that a pack can pick up a good amount of water weight in the mesh and fabrics. Something like my dyneema and silnylon ULA pack, I'm not too worried, but I do still have padded shoulder straps and a hipbelt to consider. Some of these other really cushy packs with padded backpanels and stuff, I often wonder just how much water weight is added to the pack after a good soaking. I mean, sure, you could use a pack cover, but again, one more thing to carry and weight to add, and it doesn't really protect your pack picking up water where it's most prone: back padding and straps/hipbelt. I've used pack covers with rain jackets, and the water just runs between your pack and your back. So, if I were to drop the poncho for backpacking, I wouldn't use a pack cover anyway, because I don't feel like it's worth it if one uses a pack liner or dry bags to protect the gear IN the pack. Is this protection against water weight enough to justify the negatives of a poncho? I'm not sure, still working that one out for myself. But I feel it's worth consideration. For day hikes, I just let my pack get soaked because the added weight wouldn't matter given my already low day hike loadout. Washes off the dirt somewhat anyway
I guess in sum, I would have to say:
Day Hikes in clear or light rainy weather: Yes
Backpacking: Leaning towards No, not sure yet
Dedicated Rain Gear: No
Dedicated shelter when rain is likely: No
Shotgun, I'd think for you, in the rainy PNW, some solid rain suit would be the way to go given the amount of time you're likely to be in the wet. You're more likely to HAVE to move in the rain, and for that I think a rain suit wins.
ETA: One last note, for cost, weight, and durability, I think a silnylon poncho is pretty good. One thing holding me back from getting a good rain jacket and pants is the cost for the more durable options that also have better breathing performance. My Silponcho is for sure more durable than my DriDucks rain suit, and weighs nearly the same. I would actually chose to walk through brush with the silponcho rather than the DriDucks... I'd love to pick up some eVent stuff or whatever new stuff performs well these days, but man, the cost takes some saving up for...
And for day hikes in definitely clear weather, or small chance of rain, I often find myself more inclined to to take a cheap disposable plastic poncho and an actual tarp. Now if only I could find a relatively durable 8x10 or thereabouts tarp that's just as light as cuben, but transmits less light, for an affordable price...then I'd be set. It'd work as rain shelter, wind block, and sunshade while being light and lasting for a good amount of time with common sense care.