Er, Ben, which part of the PNW are you in??? I lived in Eugene, OR for ten years and all I can say is that it is the most humid place I've ever lived (and also my favorite place to have lived! The summers are very dry, yes, but the rest of the year it just rains, rains, rains, rains! Not hard, torrential rains, like Japan, but endless weeks of drizzle that never stops. You go walk in the forests and absolutely nothing is dry. I'd say the TarpTents work perfectly there, because the rains are not so hard, and the ventilation keeps the tarptents dry inside, but I certainly wouldn't say that the PNW is not humid.
The west coast of BC, Canada also rains a lot. You can expect weeks of rain on the West Coast Trail and Juan de Fuca Trail. And lots and lots of mud. But the TarpTents still are a good choice, I think.