I almost always turn motion blur off, In real life objects don't blur for me when I move fast at least not as extreme as in games, so it fells kinda unnatural, at start seems fun like when you play for 2-3 hrs but after that it becomes annoying , plus it sometimes causes motion sickness which can lead to headaches.
There is actually tons of motion blue when you move your head fast, but your body has natural ways of masking and minimizing its effect. If you are able to keep your eyes open without focusing on anything, and shake your head back and forth, everything will be heavily blurred.
http://en.wikipedia.org/wiki/Motion_blur#Biology
When an animal's eye is in motion, the image will suffer from motion blur, resulting in an inability to resolve details. To cope with this, humans generally alternate between saccades (quick eye movements) and fixation (focusing on a single point). Saccadic masking makes motion blur during a saccade invisible. Similarly, smooth pursuit allows the eye to track a target in rapid motion, eliminating motion blur of that target instead of the scene.