I realize that with the likes of Evlesoa etc in this thread that replying with a serious answer might be a little late but I do want to point out one big difference between cinematic frames and computer generated frames -- motion blur.
If you pause a movie frame, you will see (if there is movement) that you're not really seeing the objects in their location at some slice in time, you are actually seeing the result of their motion during the frame (1/24 seconds of motion) composited together. This gives your brain motion cues.
A computer can not generate motion blur without heavy processing -- in Tremulous this is not done. Thus a frame from your computer is a snapshot in time. What you see is the exact position of objects at some point in time. Technically this is incorrect because you should be seeing a composite of motion during the frame duration. Thus if we were to set com_maxFps 24 and play the game, it would look like a slideshow because none of those frames are giving your brain motion cues.
So now you ask, why would, say 72 fps be better than 24? Like you said your eyes can only distinguish individual frames if there is at least 1/24 seconds of time between them. What happens when we (at 72 fps) show the eyes 3 frames during that time interval? The final image the eye sees is a composite of all three frames. If the object is not moving too quickly, this composite can look pretty close to what a real motion blur would have produced.
A completely different topic (but since you mentioned it) is why would someone run 300 fps? Because they're stupid.

Your monitor has a maximum refresh rate which limits the number of frames it can display per second (about 60 for a flat screen, higher for a CRT). If vertical sync is enabled, the game will match its rendering schedule with monitor refresh times so that each frame is drawn completely and only one frame is drawn per refresh. If vertical sync is disabled, the game will render frames as fast as it can (burning CPU and GPU), drawing multiple frames in one refresh. However, you only see the final result of that process at each monitor refresh. So your computer will have calculated maybe 3-4 frames per monitor refresh, but you only see one and you are very likely to see only half of the final frame which results in
tearing.
If you didn't get any of that,
read this.