"And trekking poles are for skiing, not hiking or setting up shelters people.
And you stove whackos out there, beer cans are for drinking beer not cooking dinner.
Innovation has no place in the lightweight backpacking community. These are the rules."
Excellent point. Not to mention that Cuben was made for sails on sailboats.
Plus nothing can stop one from modifying and/or making improvements on gear, bivy sacks included. As some of you may have seen I have been preparing and debating the variables of my new tarp/bivy shelter system. As of now the whole thing all in weights 355g for total enclosed protection from elements and bugs/critters.
I think once the issue of condensation is either solved or made minimal enough to not really be an issue for a given bivy, what a bivy brings to the table for backpackers, especially UL ones, can't easily be ignored. Aside from the potential for a lot of weight savings, a bivy also provides a bump (albeit a small one for most bivies) in warmth. It also blocks drafts/wind, rain spray from the edges of a tarp, dew/mist, and also protects your sleeping bag and sleeping pad (provided your pad fits inside, of course).
Of course there are subjective preferences--some people don't like being in small spaces, etc. And yes there are more objective drawbacks, for example one can't sit up inside a bivy and say cook breakfast, i.e. less space.
But please, let's not resort to things like a genetic fallacy. Let's talk about specific issues with specific bivies for use in specific conditions.
For the record, own two bivies: a Borah Cuben bottom M50 top, and a Ti Goat Silnylon top (I forget the bottom, but it's waterproof and not Cuben). The Borah one I just got and have not tried out yet. The Ti Goat I have used around 8-10 times in various conditions here in Sweden/Norway. I have only had minimal frost or condensation in this bivy, and only in the foot box. And the climate here is fairly humid with a lot of rain.
Now all that said, I actually agree with some critiques expressed about certain higher weight bivies. If you ask me, 26oz is waaaaaaay too heavy for just a bivy, even if it is supposedly a stand alone shelter. Personally, I think if your tarp/bivy combo goes over 500g, you might want to think about using a hybrid tent that is roughly the same weight, like the SMD Sky X. If you are strapped for cash, then I would extend the tarp/bivy to a full 1kg, which is not too crazy heavy, and you can get a big and good quality silnylon tarp for a reasonable price (especially used e.g. Gear Swap). And you can make a very cheap MYOG bivy out of lots of different stuff, but here is one very cheap and seemingly effective solution that claims to be 5 bucks of Tyvek/tape and around 10oz (skip to 8:30 to get to the bivy):
I know I am late to the party and that the author of the OP is essentially "done" with this thread from what I gather, but perhaps others in the peanut gallery will find the above helpful--if anything to learn about the genetic fallacy:
EDIT: Typos, always typos...