Despite what some people think, there are parts of Utah and Arizona that get chilly at night. In addition, even when it is hot, you will be running the AC. I'm not sure about you, but I like to use a blanket when the AC is on as it tends to get a little chilly after a while.
If you do not want to buy light weight sleeping bags, I'm sure you can get cheap sheets and comforters as well, but I suspect that might end up costing you more money than the sleeping bags we bought.