So, Andrew Price mentioned, in a random tutorial(can't find it atm), that Cycles will render all geometry/particles, even if they are not visible to the camera; This suggests that Cycles doesn't use a frustum check, that is, check whether or not(yay redundancies!) an object is in view.
I would assume using it would improve performance, since all games do it, so: Is it true that it doesn't use it? Why doesn't it use it?
I can actually see a potential problem, a reason not to use it; That would be Global Illumination, where objects out of view would create bounce lighting. Is this why?