It all comes down to MOTION BLUR people. This has been discussed again and again. Forget about what the eye can distinguish. The problem with computer generated 3D these days is the fact that each FRAME is a DISCRETE image. All the edges are sharp. So, the FASTER the framerate, the better, because the eye does in fact notice the "smoother" movements in the higher framerates. Now comparison to T.V. is USELESS because the camera is picking up continuous movement in the real world and basically sampling that. 1/24th of a second allows for many faster moving object to have a blur on their edges. When replayed at 24FPS, it gives a much better illusion of smooth movement than a computer 3D image at 30FPS or even 60FPS
Now, another issue is that with FPS games, the world update is set to the framerate. So, if you are flicking your mouse around quickly and your framerate during that time is 30FPS, the "granularity" of your movement will be much worse than 60FPS. The higher framerate helps you to hone in on opponents with precision weapons such as the railgun more easily. This is why hardcore FPS freaks turn off eye candy to get their framerates as high as possible.