Hi Michael -- I've spent countless nights in the desert, generally with several members of my hiking club. We've been to Dark Canyon and other canyons in the vicinity, as well as locations throughout CA, NV, AZ, and OR. The vast majority of those nights were spent outside a tent and under the stars, which are absolutely incredible in the desert.
I don't know of anyone who's ever had a problem with creepy crawlies in 25 years, apart for myself. I was stung -- exactly once -- by a scorpion in Grand Canyon -- but it was obviously not fatal and not painful enough to keep me from stargazing. It was a hot night and I was lying on top of my sleeping bag at the time, so perhaps you could take an ultralight liner instead of a tent and use it when it's too hot for your regular bag. Other than that I've never even SEEN a scorpion and, if you don't go turning over rocks and putting your hands in places you can't see, you shouldn't either.
I've never had anything crawl into my gear, though I've gone on day hikes and returned to find that ravens had opened the zipper on my backpack and stolen my cookies!
On the other hand, it can get VERY windy in the desert at times, but if you're happy with your tarp skills, I don't see a need for a tent.
Whatever you decide, best of luck!