During my undergrad, I worked for wilderness programs in southern utah. We never took tents, but usually pitched tarps, and slept on ground cloths underneath.
The tarps were usually unnecessary, as it usually doesn't rain. But who wants to wake up in the night and pitch shelter on the chance it does? Also, if you're in canyon county, often campsites can be located under overhangs, etc., and so no tarp is needed.
As for scorpions, I spent a total of 12 months in the field over a number of years, and witnessed only one scorpion sting. The hiker had left his pants out of his pack overnight, and pulled them on the next day with a scorpion in the crotch--youch!!
We always slept in the open (i.e., no headnets, no zippered closures, just in our bags under the tarps), and never had an issue with ants or scorpions at night. We always said "ants go to bed at night". I'm not sure if that's true, but they didn't bother us. Although I know scorpions are active at night, we never had a problem with scorpions at night either.
I'm taking my kids down the Escalante this summer, and I'll probably pitch catenary tarps if were not under an overhang, and use BMW style bivies over our bags, which will give us the option of zipping the bug netting closed if needed/wanted.