I live in an arid climate and the whole bivy sack thing just doesn't quite make sense. If it's going to rain, I can use a tent. If it's not going to rain, I can just sleep under the stars. But I bought a bivy sack because everybody said you have to have one for rain splash if you sleep under a tarp. Really? I have an 8x10 tarp and it appears I can scoot far enough in to avoid rain splash, if it ever rains, which it almost never does when I go out. So anyway, I figured I could use the bivy sack to add a little extra warmth for my quilt in the mean time. But that doesn't seem to work all that well. When I roll over I can still feel the gaps. So last night I decided I could try using it as a bag liner. So I tried that and the bivy sack worked well. I never felt any drafts when rolling over. It didn't feel like it was in the way. Granted it was not that cold of a night--my water didn't freezer or anything--but it seems I may have found a good use for the thing. And if it ever does rain on me, well then, maybe I'll find out what all this rain splash is all about.

Piper,

With a 8' X 10' tarp you really don't need a bivy. However is you are using a 5' X 8' poncho/tarp, the bivy is very much needed in bad weather. Also the poncho/tarp doubles as rain gear.

A properly sized and set-up quilt shouldn't have drafts.

Also, using it as a bag liner might create a vabor barrier effect, depending upon the material used in its contruction. Bivy's can get really tricky.

Noting that you both are from Cali, I think this might be a regional thing. Where I live we get over 150 inches of rain a year.

Yes, it's a regional thing. I don't see a need for a bivy since I rarely hike outside my region.

I disagree that a rain poncho can double as a shelter. I believe it can do one or the other but the only way it can do both is if your surprised by the rain. Otherwise, you are left standing out in the rain fussing with attaching the tie-outs and tying off the hood to set up the shelter, or standing in the rain fussing with removing the tie-outs to make a poncho. It's a nice idea that doesn't really work that well if it's raining enough to actually need protection.

If there's a chance of rain, I bring an umbrella and rain pants and a better shelter.

Poncho tarps are great! No need to get wet either-a windshirt with a decent DWR (like a Patagonia Houdini) is plenty of protection for the short time it takes to put the guylines away.

When you add in the fact that most ponchos have better coverage and and lower weight than traditional rain gear, it's a hard combination to beat for weight efficiency.

Piper,

You hit the downside of poncho/tarp as rain gear and shelter... as it cannot be both at the same time. If the rain is bad, I put on my windshirt which is water repellant, quickly set up the shelter... this only takes a couple minutes, and there are times I get soaked. But you can quickly remove the wet clothes, hop in your bag/quilt and cook something warm.

In the morning, it is much easier. You pack up everything while under the tarp. Then you take down the tarp and slide into it at the same time, and don't get wet at all. Mini-biners at the tie out points make this really easy. Of course don't forget to collect your guy-lines and stakes before leaving :)

Remember most poncho/tarps are smaller than your 8' X 10' tarp (usually 5' X 8').

It just takes practice.

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.

Regarding:
"Where I live we get over 150 inches of rain a year."

Yes, if you tend to get stuck in multiple days of blowing snotty rain, a larger tarp combined with a bivy would make life more tolerable.

An 8x10(or whatever) could be pitched higher with more space, when combined with a bivy. The bivy would allow a higher pitch as spray would not be such an issue.

Pitching a smaller tarp to the ground would make sitting up impossible.

Of course, if rain isn't a constant on a hike, I use a 9x5 with a bivy. The bivy is more for bugs and ground sheet than anything else on those trips. It also adds some warmth by reducing drafts.

Or you can use a pyramid tent with no floor

Have 2 people in a 4 person tent