From my point of view, most games like to keep at 60 fps for the reasoning that during some situations the frames will drop a fair ways, and at which point become noticable, even though the human eye 'could' do with less, a scene running at lower fps will look smooth, so long as it is consistent, it can still be considered 60 frames a second, even if some of the frames have a longer delay than others, that change in speed will be picked up by your eye, not necessarily the 'slow' frame rate. So trying to keep the frame rate a ways above where it needs to be, means it's less likely to slow enough for a frame or three to be noticable, if they could guarantee a specific framerate(30 or 40) without any slowdown at all in between, your eye probably would not be able to see any of the 'frames'.