I think of it this way, since a great number of rehab patients will be victims of head injury, spinal cord injury, stroke, you need to have a solid knowledge of neuro as a PM&R in order to understand that patient's injury and how to go about planning their rehab. As far as diagnosing a neurological condition, offering medical managment of said conditions, that is the neurologist job.
Now, where I am hazy is on the whole EMG/NCV thing. Seems as if both Neuro and PM&R docs are trained to do these and in some cases, a PM&R doc could potentially earn quite a referral practice of doing these studies. Now i have heard all kinds of opinions about this, I have heard some neurologist state that PM&R docs do not get the training they do, SOme PM&R docs are good, some are bad, etc. etc. SO perhaps some others can shed some light on this subject.