I have the rod mirror in the centre and it does restrict visibility quite a bit on tighter left hand bends unless you alter your head position.
I don't have strong views either way on having a central lookbackascope, but there's a very good reason to move your head when looking. If you just flick your eyes from position A to position B, your brain blanks out any image being processed while the eyes are moving. If on the other hand you move your head, the brain will provide you with images whilst you are moving.
You can try this yourself if you wish, but it's an important part of training pilots apparently. "Move your head or you're dead".
You're right about the pilot training although I never actually heard the "move your head or you're dead" phrase.
However, the circumstances are completely different.
When flying you are taught to scan the horizon for possible threats in the far distance. Many times these can still be a dot on the horizon set against a very grey background and would be very difficult to spot with a casual glance.
So you systematically move your head across the horizon and up and down depending on the field of view (including sideways) and pause for a few seconds to allow the eyes and more importantly the brain to take a picture.
If the eyes are constantly moving the brain will not have time to take in the shot and will miss things.
Why is this different to driving? Well, if you scan like that in the rear view mirror you will probably crash into something in front.
In flying, if the aircraft is set up correctly, you have time to adopt that type of scanning pattern but in a car on busy roads you don't have time to dwell in the rear view mirrors and a quick flick is all you can afford.
If that quick flick is in the interior rear view mirror you will miss a lot of what's going on in the overtaking zones slightly to the side so I always use the side mirrors and have a quick glance over my shoulder. Which as an aside is why I like hoods with the rear quarter light.