I agree - this is definitely one of the better BPL threads I've read in awhile. Perhaps similar in content to the super-informative Richard N. CLO threads from a few years back.
I hike in the high Sierra during non-winter months, but near/above 10k. The bottom line for me is that I take a (slightly oversized) tarp for 3 reasons:
Since it's a MYOG tarp that essentially follows Henry's original tarp tent design, I've got netting sewn on both the front & back, but not full enclosure flaps. This means the tarp provides minimal warmth, if any, so I need to rely on MYOG (over sized & stuffed) APEX quilt.
If any of the three conditions above are met, I break out the tarp. For example, if the typical afternoon showers look like they will continue past evening, out comes the tarp. If it's particularly windy, out comes the tarp. And of course, if the mozzies are really bad, and don't look they are leaving @ dusk, out comes the tarp.
For my quilt, I used one layer of 5oz, one layer of 2.5oz & M90 as the shell/liner. The insulation & M90, which has a good DWR, is a sufficient combo to allow me to cowboy camp. In effect, this approach is similar to a bivy, except I'm missing the add'l top layer of M90/Event/etc.
(I use a Tyvek GC, so whether it is sewn to a bivy or just laid out, the net effect is similar. My quilt is oversized by around 2" on each side as to comfortably tuck under without allowing cold air to enter.)
Where I'm SOL is if it's wet (humid), windy & cold. This is the formula for condensation, which only a double walled tent can really combat. (It's the reason why the tarp is slightly oversized - to keep from brushing the wall.) While my tarp provides protection against direct rain, it doesn't provide warmth, so the dew point ends up inside. In this case, I wear my down vest in an attempt to move the dew point to the outside of the quilt.
That's the reason for the syn quilt. It may not be perfect, but there is a certain fail safe aspect. There are just too many stories of down malfunction - like they say, it only takes one time.