I've noticed the unusable benches around a campfire issue in my game as well. However, I was able to get them to work by using the moveobjects cheat. Just place them a small distance away from their normal slot. Sims will sit on them and do all the available campfire animations. The only downside to this is it looks a little silly. Especially when roasting anything, the end of the stick ends up over the ground instead of the fire. But it works! ;) Hopefully they will fix this at some point.