(warning: wall of tech text below)
Fun fact: as TW3FanApp mentioned, the effect you're discussing is not motion blur (i.e. blurring moving objects on screen) but radial blur (a simple blur effect to suggest damage or force). The latter is comparable to an effect like shaking the screen when heavy monsters are on screen; it may help immersion for some, but it can get annoying if it's very noticeable. It will very likely be possible to turn it off in TW3, though, if you don't like it.
Motion blur, on the other hand, is an effect that can help to make a game feel more fluent at lower framerates (say 25-50). If an object is moving across your screen at a particularly high speed and the framerate isn't sufficiently high to keep up (this can happen at higher framerates too - I'm sure any of you who've ever climbed a view point in Assassin's Creed are familiar with this), the movement may come across as stuttering rather than fluent movement.
However, motion blur is also often applied in order to offer a more 'cinematic' experience (since movies, running at 24 FPS, have loads of it). If used in this way, it becomes a matter of personal preference again: some people like it, others don't. Again, the fun of PC gaming is you'll be able to decide for yourself if you want to turn it on or off.