On both of the trips where I went coast to coast I encountered freezing temperatures. One was the TA in summer and during an especially warm summer. It can snow in the Rockies any time of year. Even in the east chilly nights are not unheard of. When I strip my gear weight to the bare minimum (14 pounds last trip) I still take the Phantom 45. My Mountain Hardware Phantom 45 was fine for me at 18 F with a silk liner, thick socks, and one layer of clothes. I use a down vest as a pillow and could put that on if it was even colder. I sleep especially warm though, most people tell me they would freeze in the same conditions.

A sleeping bag does not need to be all that bulky. The Phantum 45 for instance packs to the size of a grapefruit and weighs 1 pound 1 ounce.

Going without a sleeping bag sounds like a very bad idea to me. Even on a trip where it was over 100F much of the time we had a few cold nights.