"In my opinion, the performance of WP/B shells has been greatly oversold -- the product category name, "waterproof-breathable", is itself an oxymoron. My real-world experience is that they fail to keep me dry during prolong storms, or even during short storms if the fabric has been compromised by dirt, body oils, and /or abrasion, which is unavoidable on a long trip."
So, if I am to correctly understand what you are saying, it's the "B" of the WP/B equation that is failing, not necessarily the "WP". In other words, it's your own perspiration that is making you wet, as opposed to any direct outside precipitation.
"As an earlier poster said, the problem is not so much with the membranes of WP/B fabrics but with the DWR finish, which is the Achilles heel. The DWR craps out quickly and, once it does, I find that breathability completely stops (because the exterior shell fabric is soaked, so there is no where for the water inside the shell to go) and water may actually start moving into the fabric (because it more humid outside the shell than inside)."
By "moving into the fabric" you mean, wetted out, correct? Or are you saying that the water actually moves past the WP membrane? I assume that it's not the membrane that is failing, but would like to clairify.
"Andrew is right on in his criticism of WPB garments in general. If the DWR is compromised then the sweat vapor cannot pass through the saturated outer layer, however, the membrane will still prevent water penetration from the outside. That, however, doesn't help much if you are working hard and sweating (I.e. wet from the inside)."
If it's true that it's just the DWR failing and not the membrane, then I guess that's where pit zips come in handy. Of course everyone hikes their own hike, but personally I would also adjust my pace to prevent overheating. Perhaps I'm oversimplifying the solution. I've only done a week at most with tens of miles, not months at a time with hundreds, thousands of miles.