I depend on a poncho and an AMK Heat Sheet bivy bag for summer day hikes backup. If I have it together enough, the poncho can be rigged as a lean-to tarp shelter with the bivy bag under on a bed of debris or conifer boughs and a fire if needed. If I'm not ambulatory, I could just use the bivy or wrapped up in the poncho. I don't carry other rain gear for fair summer weather day hikes.
I have weathered a nasty rainstorm holed up under a tree with my poncho completely covering me. With a basic sit pad and my pack in front of me, I was warm, dry and comfortable. I could open the pack for snacks without getting a drop inside. Add radio and kick back for a break in the weather. It wouldn't be fun, but I could spend a night that way and live to complain about it :)
I think that is the way to think about any non-insulated system--- keep yourself reasonably dry and cut the wind and you can get through a long nasty night. You won't be cozy, but you'll survive. Anyone with the essentials-- some extra clothing, food, lighting, fire making tools and some basic shelter could go for days if need be.
I own one of the AMK Thermolite bivys but I have never used it. The Heat Sheet bivy is so much smaller and lighter, it has been the first choice to throw in my day hiking kit. Someday (in August) I want to try a uber-minimal summer kit with the Thermolite bivy, poncho tarp, a light pad and a no-cook menu.