why motion blur is sometimes bad

As a Harvard Neuroscientist, here's another mind-bending thought. 
When you rotate your head, you are NOT in control of the primary directionality of your eye movements. 
Try this. Keep your head still and look at your finger while you shake your finger left and right; your finger will look blurry due to motion blur.
Now, instead, keep your finger still and look at it while you simultaneously shake your head left and right, which will demonstrate - in this situation - that you are still able to maintain a stable image of your finger. 
This is because your sense of your head in space - your vestibular system - has a hardwired reflex to move your eyes in the opposite direction of your rotational head movements. 
This vestibular-ocular reflex (VOR) ensures automatic dynamic stabilization of images during head movements. 
You are not consciously doing this, and you can not consciously override it. 
The VOR is truly a reflex and, interestingly, does not depend on vision. During head movements, you will still have a strong VOR if you are in the dark or if you have your eyes closed. 
Your VOR originates from your vestibular system, which is located in your inner ear right next to your cochlea - your organ of hearing. 
Your vestibular system for rotational head movements consists of a series of three fluid-filled semicircular canals that have little "hair" cells inside the canals. 
When your head moves up-down (pitch), left-right (yaw), or shoulder-to-shoulder (roll), fluid inside the corresponding semicircular canal flows. 
This head motion-induced flow bends the hair cells within the canal to ultimately send electrical impulses to your hindbrain. 
These electrical impulses - encoding head movement - then talk to parts of your brain controlling eye movements, which connect to your eye muscles to reflexively move your eyes in the opposite direction of your head movement. 
So be thankful that your sense of your head in space - your vestibular system - takes care of maintaining a stable image of the world even when your head bobs around as you walk. Otherwise, walking would be a blurry journey of craziness. Videos coming soon. 




in video games there shouldn't be motion blur when rotating your head.
but there should be motion blur for objects moving relative to you position.