You mentioned Washington and Oregon as destinations.
I have some experience with ponchos breaking brush in the rainforests on the West side of the Cascades. The physical contact with the vegetation will eventually soak your clothing, and that will wick up eventually into your top layers even when covered witha poncho. So in that area, WP/B clothing is preferrable. In the dryer less vegetated Eastern half of those states, a poncho would be fine based on my experience in the topographically similar TX, AZ, and CA.
Personally I haven't used a poncho in years, and now that I learned about windshells here at BPL, my windpants and windshirt are good enough for light rain/wind/brush, and my DIAD and rain pants are reserved for the heavier stuff.
A poncho is multi-use, certainly, but so is a DIAD or other light WP/B shell. It cuts the wind, insulates at about clo 1, and is completely waterproof. Personally I don't want to have to take off my shell to pitch my shelter. And with a poncho you are going to carry rain pants anyway, right?