I think it's a mistake (potentially a serious mistake) to think of the new treated down as "waterproof". Think of it like the DWR finish on a windbreaker (which is what I think it really is). DWR will keep water beading up on the jacket up to a point, but it won't make the jacket waterproof. The fabric will eventually soak through. The treated down will repel a little bit of moisture, but if you get it really wet, it's going to be wet down, with all of the problems of any wet down.
Personally, I wear down as often as I can -- anytime it's dry and cold. But, if it's raining or if I'm going to be sweating, then it's always a synthetic insulation layer. I never hike in down, but will put on a down jacket as soon as I stop.
It's not quite like DWR. These new downs are treated on a nano molecular level, each microscopic plume fiber coated. DWR is akin to a thick (read easily removed) layer of heavy hydrophobic coatings.
I've been using DownTek and DriDown for a couple years now. I was given an early Sierra Designs Cloud to test for a winter and as hard as it was to do, my brain didn't wanna, I actually wore it in a full on rainstorm just to test it. It took at least 30 minutes before any water started to genuinely wet the down. Maybe even more. After another 2 hours in a drizzle, it did get noticeably wet in spots, but still insulated, and bizarre as it was, the wet down stayed unusually lofted. I then threw the jacket in the corner of my tent where during the course of a 60 degree night...it dried. It retained 100% of its loft.
I also washed it about seven times just to test the durability of the treatment to the down. Every time it dried nearly instantly, again retaining all of its loft. Later a representative from Sea to Summit said they washed their DryDown test bags over 50 times before they gave up, each time the down sprang back to life good as new. I've also found the new dry downs breath far better than synthetics which are notorious for getting clammy and retaining moisture. While climbing in Alaska a client's bag gained at least 10 pounds of moisture in a week! The new dry downs radiate that moisture throughout the bag or jacket exceptionally well.
I do agree with you, that ,water-resistant down isn't waterproof, but darn close. I just picked up the Big Agnes Meaden Jacket with DownTek, the Sea to Summit Spark II with Ultra DryDown and both have shed water like a duck's butt. It takes an intense amount of water to wet these down plumes and they dry so fast, and without damage to the plumes, it's freaky.
By the way, if you put a pile of water-resistant down in a pile on a table and spray it with water, the droplets eventually end up on the table, the down almost 100% dry. It is crazy.