I found the openness of the tarp the most appealing thing the first time I used it. I didn't know this until the first time I went camping in bear country, but apparently I'm afraid of the dark. Things that go bump in the night, to be specific.
The problem with a tent, for me, is that I hear something, wake up, yet lack the superman x-ray vision to see through the tent wall what's out there. It's probably nothing, but when I can't see it, my imagination goes into overdrive.
And it's not like a thin piece of silnylon makes me feel protected.
With the tarp, camping in Little Yosemite Valley, I woke up hearing something trodding through my campground. I was able to just glance up through the bug netting of my bivy and see that it was just a deer and go back to sleep. In a tent, I'd have been awake for the next 30 minutes listening really really hard any little sound, wondering what it was.