Sorry I missed your question in the post above. I think that you are correct. In Arizona, during the Summer or Spring/Fall situations the Clark weather shield would usually be adequate for wind protection without the tarp. The weight savings would be on the order of ~19 ounces if you delete the standard Clark RX-250 tarp and rigging from your pack.
I did have a warm night on Saturday with the combination of the Clark Z-Liner and the JRB MW3 UQ. Although, my feet did get pretty cold when the temperature hit 14.6 degrees. I got up and readjusted my gear, and eventually warmed up. Wow, I really wish that I had a pair of down booties like Drewboy's.
My son Jacob was sleeping in his Warbonnet Traveler with an SPE 4x4 carrying two, 72" Thermarest Z-Liner CCF pads. Thank you again Shug for the suggestion regarding the SPE. Jacob's top cover was a NF -15 degree down bag with a Goretex bivy sack. He zipped the system up only about 30" to form a foot box and flipped the bag/bivy to use as an over quilt. He was warm all night, with the exception of a bit of cold in the feet in the early morning.