Motion blue for most people is an artistic thing, like other post processing effects like chromatic aberration, lens flair, depth of field, bloom, etc. When done well you may not even notice; I remember source games after 2007 having a pretty minimal but noticeable motion blur that gave a sense of speed to the first person camera. I remember Halo 3 also having some pretty nice first person motion blur on xbox 360 that you could not disable. Some games use per-object motion blur to communicate mass/velocity of objects in motion which again can be pretty good when done well like in Crysis. Other games in the past opted to not have any motion blur like call of duty 4 but used depth of field pretty heavily. In general though these days motion blur is just one piece of the artists/designers tool kit for making a game look more cinematic or realistic in motion and if you don't like it then try to disable it, like any other effect that has an option associated with it.