I cowboy-camped for a total of seven nights in the Grand Canyon last summer.
I cowboy camped for a couple of nights in the Grand Canyon too, a few years ago; my hiking friends thought I was unwise, but ditto I had a poncho tarp that I put up maybe once and didn't really use. The issue I noticed was the wind, though --- hard to block it and it was a little distracting.
I also had a ringtail jump on my chest in the middle of the night; kind of a cool experience once I realized that it hadn't damaged my sleeping bag.
This is quite an old thread resurrected, but responding to the original post anyway --- what the heck! --- that post was:
Has anyone here cowboy camped frequently while thru-hiking? It just seems it would be so much of a less hastle than to have to set up a tent or tarp. I would still take a tarp with me for the chance it could rain.
I've thru-hiked multiple trails and cowboy camped relatively seldom --- several times in SoCal, and a couple of times in New Mexico, and that's about it. This is partly situational, and to some degree I expect personal style. But I'm not a fan of bugs, and if they're out at all I'm inclined to be in a tent, ditto any real chance of rain or significant dewfall. What I find happens is that I get so used to the tent that sometimes I don't think after a while about cowboy camping even if the opportunity arises. After a few weeks on trail the tent really is like home, I know exactly where everything is in the night, nothing is going to go rolling off or underneath the ground cloth, and I spend a little less time in the morning making sure that I have everything --- especially for those times when I'm packing up before the sun is up.
But I know that people do cowboy camp a lot more than I do, and I've quite enjoyed the times that I have.