"I still don't understand that one either."
Its not that hard to understand or figure out - *full* enclosure keeps out mosquitoes and ticks and other potentially problematic insects (tic protection is very important in certain places like Ventanna wilderness where I go a lot). Condensation will roll down the tarp then drip *through* screen at the edges, so will almost never will be a real issue almost no matter what. IMHO, based on my experiences with both issues a smart and novel solution. Best of both worlds. Not the only one, but very creative. Plus you can choose your groundsheet depending on the conditions you expect, or if you will be sharing the shelter with someone. Or you can just do cowboy camping when condition are right using the *removable* groundsheet. About as good as it gets in a single shelter. That kind of flexibility is worth the extra couple of oz to me. I love mine.
My twin ground sheet is 4.2 oz not 5-6 oz.
Also FYI, the screen isn't really lighter than the cuben, so that was never the point.
The zpacks ground sheets have loops in them already, so even without the rain poncho version you could probably rig up something creative as far as rain protection or pack cover if that sort of thing floats your boat. Sun awning for afternoon stop..and so on.
Joe has done a triple crown, and some other long distance stuff, so I'm guessing he may have learned a thing or two about what works, so may not be the smartest decision to dismiss the idea off hand just because it seems different on first blush. A lot of people who have actually *used* the design appreciate it quite a bit. They don't have such big following and a long waiting list for nothing.