Just a total guess here but if you're using GI, then the objects which are not in the camera view are still computed for lighting.

So, if you "turn them off", you remove them from the GI calculation.

Now none of that matters if you're using a saved map for your lighting.

Another question is if you have ANY kind of reflective surfaces in the scene.  If you do, then C won't fuss with all that reflection calculation.

And last but not least, C still has to do some sort of sorting of objects to determine what is and what is not in the camera view.  If you have a gazillion objects that are managed via a replicator, C still has to compute if those things are in the view for every frame.

So, I'm not completely sure that it's a bug.  Regardless, it's a good tip even if it is a bug.

Just some thoughts.

-Kix