I've been looking at tarp shelters to include in my BOB, and now I'm wondering why I would want to go that route rather than an inexpensive two man tent like the REI Trail Dome ($99). Reasonably sized tarps of decent quality (not the blue tarps from hardware stores) are not cheap. Even if you go for the lower-end ones. Other than maybe 2 or so pounds of weight savings, I can't find much reason to justify a tarp setup for BOB use. A tarp would give less protection and present a more complex setup most of the time. And dealing with shifting wind patterns with a tarp could be tricky. I fully understand why someone would choose a tarp just for the weight savings aspect. But are there other reasons? I'm thinking, just buy the REI 4-1/2lb tent and be done with it.

That tent looks like a great deal.

Some reasons you might like a tarp.

Some tarps are much more compact than a tent if space is at a premium in your BOB. A tiny sil tarp can be about fist size.
It will even fit in a pocket.

2 something pounds would be a lot of
peanut butter, whiskey, or 22 ammo.

The prime reason for selecting a tarp over a tent is weight savings, which can be significant over the course of a multi day trip. They are not particularly difficult to pitch and can offer surprisingly good protection.

A worthwhile compromise is selection of a tent that can be pitched with just the fly, poles, and ground cloth - more protection than a tarp and easier to erect, but lighter than the full tent. In bug season, just carry the tent - you will be glad you did.

I look at this decision differently for BOB than for weekend backpacking. I choose tent for backpacking, poncho/tarp for BOB.

My BOB has to get me a much longer way in much less time than if I'm recreating. If I'm bugging out then comfort shifts lower on the scale. Weight, which equals speed of movement, becomes more important.

Of course if I knew ahead of time that the bugout was to destinations unknown forever then I'd make different choices.

Other advantages to the tarp vs 2-man tent for non-recreational use is the ability to set up shelter in irregular terrain. A tent needs a specific footprint of clear and fairly flat ground. A tarp will work in a boulder field, urban rubble or a forest of downed trees.
It is also pretty handy in a sudden hail storm or passing squal to just grab out of your pack, drape over you, your pack, your dog and a friend or two.
It is easier to rig your shelter to collect rain water for drinking.
However I don't think I would have survived in Denali Nat'l Park without a bug-proof tent.
leo

i simply prefer tarps because i like being outside, that why i'm camping. i enjoy viewing an open fire next to it and how it warms me. i like instantly seeing what's going on around me. they dry out faster. they cover more ground area for the same weight/packing-volume. they're safer if a stove needs to be used under one. i find tents to confining and depressing, nor are they as easy to exit should one catch fire.

but tents can be the better option in treeless terrain, extreme wind conditions, or buggy areas...

I guess part of the question would be what is the planned purpose of the BOB? Is the plan for the bag to be used when you definitely need to sleep outdoors or is it more of a low-probability event that you are planning for?

While I don't really have a BOB (so maybe take this with a grain of salt), I think this is a good way to look at it.

For example, if I'm backpacking I will choose the tent over the tarp. I prefer the comfort, weather protection and privacy it affords.

If, however, I am not planning on staying overnight when I head to the woods I would bring a tarp (or, for me, something that could be used as a tarp) as a contingency. I would not want to pack a tent for a just-in-case scenario.

So back to the BOB, if most of the events being planned for would require staying outdoors I'd lean towards including the tent if the size and weight weren't problematic. But if most of the planned events wouldn't require sleeping outside I'd likely go with a tarp or equivalent (those space blankets I linked to are a great, multi-use item that fit this bill without breaking the bank; they are about $15).

Thanks for all the replies! Denis asked a very good, probing question. "What is the planned purpose of the BOB?" To be honest, I don't anticipate sleeping outside. So my shelter needs are of the "just in case" variety. So I agree that does favor a tarp. I found an inexpensive one that might work, the larger 11'x11' size here: http://www.sportsmansguide.com/net/cb/guide-gear-11x11-tarp-green.aspx?a=954701&e=Y

I'm sure this tarp is mostly junk, but it does have more and better (sewn) tieouts than the typical cheap grommet blue tarp. It's inexpensive, and might be serviceable for my "just in case" requirements.

Depending how resourceful you are it is possible to sew up your own.

Be aware that the dome tent is not a 2 person, more like a 1.5.

Equinox has decent inexpensive tarps.

A third option is a bivy bag. About the same weight and volume as a medium sized tarp, it can provide more weather protection.

A fouth option is a Hennesy Hammock.

A fifth option is a tarp-poncho.

Each of these options are single user only whereas a tarp of similar weight can provide shelter for 2 or even 3.