I have used my cable lock at campgrounds. I also sleep in a hammock and have a big enough tarp that sometimes I just bring the bike under tarp. It's not usually locked, then, but I have it in eyesight (except that my eyes are closed). But sometimes I don't even bother. It just depends on how paranoid I want to be. Stayed at Ohiopyle State Park the other night and just left it unlocked. I just had a hard time imagining someone grabbing my bike and escaping that place with it in the middle of night. I had a hard enough time getting in/out in daylight.
Really I'd say I leave it unlocked at my site more often then not, but if there are any circumstances that activate my paranoia, then the cable lock is really all I need. And since I'm in a hammock, I can generally lock it to the tree I'm in, which means that anyone trying to mess with it will have to right next to my tent.