Those passing, middle of the night rainstorms are what make me reluctant to camp sans-shelter, using my tarp, ala Mike C! as a pillow. I make end up bringing the down quilt since I do have my synthetic BPL cocoon hoody as a safety layer if my down got completely drenched, which is unlike anyways. It would be survival level-safety, and I wouldn't likely sleep, but I wouldn't die either.
The first two nights of my trip, according the the 7 day weather.gov forecast, are looking clear with a low of 34 and 37, respectively, perfect for sleeping out under a tarp. My BPL Nano is only slightly smaller than the SpinnTwinn, (9" at the ridgline, 8" head, 15" foot) and I did okay in rain and hail, probably didn't even need the bivy, which in fact probably caused more condensation wet-out than I would've gotten from spray each night, though it does scare me to leave the bivy behind when bringing the Nano, which would make the Solomid/Nunatak combo the lightest by about a 1/4 lb, but the Nano/Nunatak/bivy combo has the life-style appeal of sleeping out/open views. Probably I'll pack both and then leave one combo in the car based on a more timely weather forecast.