A screenplay describes not just what actors will say, but the setting for the scene as well as action. Slugline doesn't cover it fully. It seems like a cheat to me, or laziness not to let the reader know where the scene is set and the nature of the environment.

To be fair, that Heywood example comes in the midst of a sequence, right? We know that Heywood is just in his cell looking out into the cell block. I know the point of the question is clear-cut scenarios, but that lack of description isn't in a stand-alone scene.

Also, you want to be wary of joining static scenes (unless you're making a point about there being no movement). Your cuts need to be dynamic - so if you can't think of anything useful to say beneath the slug, perhaps ask yourself if you're joining the scene at the right time and in the right way...