Quiet Storm,
This is going to be one of situations when "camping" or "hiking" is going to lead to different gear choices then a BOB survivial situation.
I am a long range, super light weight hiker. I often use a tarp for a shelter in all four seasons. But doing that, I almost always have the time and ability to pick my sleep spot in daylight and do some enviromental prep. For example, get myself a bed of pine boughs or find a convienent wind breaking rock or sand trench. I have even dug out a snow cave and spent a fairly comfortable night in it with a small candle, wrapped up tight in my tarp, surrounded by some evergreen boughs.
In a survival situation, I will not have that luxury of time or energy. If things are really really bad, that enviromental prep can lead trouble right to you and you might find yourself hunkering down in some poor campsite choices where prep is just not going to help.
In that situation, I would want my bivy sack. Fast, light, reasonably water proof,warm, low profile, keeps the bugs out. Also, I can use it wounded, which is why it's on my long motorcycle trip packing list. If I come off the bike in some of the beautiful and desolate places I ride, at least I have a shelter I can drag my sorry self into with zero prep time.
My particular bivy gives me about an additional 22 degrees of warmth compared to an open air camp, and has a bug net over the face. Sleeping in such tight confines takes some getting used to, but that's just a training issue.
Most of the expensive bivies have spectra fiber loops for harnessing into on multiday technical assents, so you can even pick vertical sleeping spots if you have to.
Just my two cents,
Jeff