Yes, of course, you are right. The sphere isn't imported, unless you rename it, I presume, although the image itself can be rendered.

I use the cube mainly on open sets such as you describe. I have one made to a useful shape, size, material and location and saved as a prop. I can easily re-size its various dimensions to suit. I much prefer these semi-open to rooms as you have so little elbow room when setting up—in some cases there's hardly room to swing a mesh light. Yes, you can turn things on and off or make them transparent, but it ends up being very fiddly. It drives me nuts. On the other hand, I rarely do long or wide angle shots so partial sets suit me very well.