Here's an oddball question:

Does Motion 3 render as it SHOULD, that is, only what's in the frame/camera view in a given frame, or does it render EVERYTHING, even stuff outside the frame/camera view (i.e. that which a viewer would never see), even though it throws it away afterward.

In other words, do I need to keyframe visibility of layers when they're outside the frame/camera view to keep them from rendering?

Why would I have something outside the frame? Using 3D camera to scroll past a background, for instance.

I can't speak to the specific of Motion but in general, 3D drawing packages have a well defined drawing pipeline that takes the internal graphical model and turns it into a 2D rendering. The rendering pipeline includes the window. While internally, the model is drawn in a large 3D "world", when that world is mapped onto the 2D window, anything that's outside the window is thrown away.