Wait are you complaining about the accuracy of movement direction or control over rate of movement? I doubt it's the rate of movement since I can't think of any FPS games where I would actually need or even want fine speed control.
So that just leaves directional accuracy of keyboard movement which is heavily dependent on mouse accuracy since all WASD movements are relative to facing, and facing is of course controlled by the mouse.
If you agree that aiming with a mouse is the most accurate, then it's a small step to realize that movement derived from that accurate aiming system is also extremely accurate. If you want to adjust your heading ever so slightly you simply adjust your facing. This works well because in an FPS your movements will fall into one of two categories 1) towards/away from something or 2) parallel to something. WASD provides both categories, while the mouse allows you to adjust what that "something" is with a high degree of accuracy.
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| To Each Man, Responsibility |
