If shutter speed has been neglected in all the talks about manual control, what to say about the ISO? Would it not have been in the firmware change log, I doubt many people would have cried. At least for the first few days.
While aperture and shutter speed give creative control over the way you present a scene, the ISO impacts the overall look. Switching from ISO 100 to 400 might be a no brainer if you need more light when taking pictures. On the other hand, if you are editing a clip and interlinking scenes shot at various ISO, the differences will be obvious and viewers will expect it to be done for a reason.
Look consistency in a scene is king and having control over the ISO helps keeping the same look from the start of the scene to the end. This part can already be done with the exposure lock button (*). The new thing here is that you also want to set a predetermined ISO in a case where you have a multi camera setup or in scenes where lighting changes drastically.
Some people might argue that most of the look can be generated during the grading stage but, as with everything, it is always better to do it in camera than in post. You will save yourself a lot of time trying to match clips with each others.