Forgive me if I'm saying anything obvious, as I'm from central Texas and since we don't get much interesting weather down here :-D, I haven't much chance to observe how water does/does-not penetrate the DWR on my various shells.
But last night it was coming down, and I had some free time, so in a dorky move that I'm sure a lot of us are vulnerable enough to admit . . . I went outside and stood around in various different technical pieces that I own, taking note of their breathability and water resistance. Usual drill.
Now what I took note of for the first time was that when it was sprinkling, I could stand out for a while in my OR Ion or my Cloudveil Prospector, and water would bead up indefinitely on the surface of these things, hardly ever wetting through. It seemed like no amount of sprinkling would break the DWR.
When I moved under the leaky gutter 20 feet up that drops a heavy stream of water, a proper miniature rainstorm, both of these items wet through immediately. Not within a few minutes or even a few seconds, but immediately. The DWR seemed no match at all (which could be because I've had both of these pieces for a while . . . but maybe we can ignore that for right now?).
So, what I'm wondering is maybe DWR efficacy is based NOT on the AMOUNT of water (since I can run a medium faucet stream over either the Ion or the Prospector and it will continue to run off pretty nicely) but the VELOCITY of it. This makes sense to me initially since everything I know about real waterproofing is based on pressure, and of course a faster moving rain droplet will create more instantaneous pressure on impact, yeah?
If this is true then I should rethink my clothing systems a little bit. I've been influenced a lot by marketing and by the threads and articles that I see here, and had the impression that if it was going to rain for a long TIME on a trip, I should take a little something more than a wind shell. But now I'm not sure this is true, because my (maybe mistaken) impression is that if it's misting or sprinkling for a long time, these lightweight DWRed pieces will work just fine. And the only reason to take a proper waterproof piece is if the rain will be heavy (and of course, sustained enough that I can't dry out the layer with my own body heat).