I've been experimenting with multiple use sleeping systems that include an ultralight synthetic bag (50F) and lightweight insulative clothing. Last weekend I went with some friends on a two night, three day hike on the A.T. in Vermont. I carried no bag at all, just a lightweight fleece jacket and pants. Though a little warm around camp in the twilight hours, they were perfect in my Hennessy Hammock with a 3/8" foam pad (with wings of padding duct taped on for torso) through the night. I used a Golite Wisp pullover to keep my torso warm enough the first night, when temps dropped into the low 50's. It can be done, just don't push it. I figured I'd be safe, since the weather forecast was clear and warm weather through the trip, and I had friends to borrow a jacket or other warm items from, if needed. If going solo, I probably would have brought my Golite Coal jacket, which is a bit warmer than the fleece.
The beauty of this system, especially for hammock users, is that you can get up, do chores, tighten lines, visit the woods (ahem), etc, without having to wriggle out of, then back into, a sleeping bag. I also have a Feathered Friends Rock Wren, which I'm fond of for cooler weather, as the drawstring bottom allows you to pull the bag up to your thighs and walk around, and the front opening zipper allows you to zip down past your waist for nighttime trips to the woods to intentionally dehydrate.