I really like the size of a poncho tarp. If I had more money, I might buy a tarp the size of a poncho. But the 8x10 is what I have and I like it well enough. It's so versatile. I often just fold it in half and use it as a lean-to. I'll bring it when there are two of us and use it full size. And if there ever was any real weather, just me alone under there would be sufficient protection, no bivy sack needed (saving 6oz of weight.)
I bought the bivy sack because I had planned to hike a portion of the PCT with it and my rain poncho. It only rained a little bit, not enough to have any trouble with the whole system. But the kind of torrential rain we get in So Cal would make the whole system pretty annoying. And if it was torrential rain, I'd really rather have a larger shelter and better rain gear.
So, as an emergency system, where you really don't expect rain, the whole poncho thing works fine. But if you're hauling around a bivy that's not being used, here's a way you can use it and get some extra toastiness out of your sleep.