I'd never plan for more than 40 deg night time lows in the smokies in the summer. There are plenty of nights where it might be 50+ but as with any hiking locale, with elevation comes uncertainty.

At lower elevations, temps might be warm, and a liner bag might be enough, but on that same night at 5,000 feet, you'd probably be shivering.

Last time I was up there I had a crappy 40 degree synthetic bag and never got a hint of chill, but there are plenty of times you could be pushing it pretty close with a 30 degree bag.

Now I have a decent 20 deg down bag (really more like 25-30) that I feel I can trust for most 3-season, with the real deciding factor being what I sleep on underneath the bag (z-rest during decently warm weather, BA ins. aircore in cold).

I'm sure I'd be more inclined to push it if I had a wider selection of bags, but as of right now I'm down to the one bag that serves most of my needs...until I pony up the coin for a summer bag (eyes on a summerlite from WM).