The Military Spec Ops teams use tarp shelters because of what you just said...they can see out of them and fight from them. They just roll out of their shelter into a pit they dug next to the tarp and are able to fight. Tarps offer much greater advantage for visibility and ingress/egress. Also low noise discipline with no noisy flaps, doors, zippers/velcro.
Yeah, tis true. Don't know about the rolling into the fighting position. On patrols, we didn't use "tarps" much other than to wear them and perhaps grab a snooze while waiting at an ORP or other stop if it became an extended stop. (50% security at least!)
Our "tarps" were ponchos. We often carried a couple. We used cammo'ed bungy cords to set them up quickly. I could get my "hooch" shelter up in a matter of seconds.
The reason I still like camping this way (besides the not to underestimated effect of personal inertia/"I've always done it this way") is that the "tarp" can be anything. If it's warm and I just want a rain fly with a breeze, no problem. If it's subfreezing and a I want an enclosed diamond with very minimal interior volume, I can do it.
And if I'm using my poncho as the "tarp", when it's no longer a shelter it can be my raingear. I can't wear my small tents as raingear. The poncho/tarp can also be used to make a gear roll, keeping my stuff dry and allowing me to carry it with easy. I can't use my backpack/shoulder bag as a shelter, but I can use my tarp in place of my small backpack or shoulder.
There are lotsa other things a tarp can do, ie make a litter for an injured fellow, more easily adapt to natural shelter or shelter you found (plane crash/old damaged shed etc), collect water. The list is probably endless. All more easily with a tarp/poncho than a tent (unless you are willing to cut your tent).
I feel the same way about a poncho liner and/or wool blanket vice a sleeping bag. It can be my sleeping bag. It can also be worn under my poncho in extreme weather. Can't do it that with most sleeping bags.
