its what your eyes and brain is used to, if you only play at 30fps it will feel fine.
I do think people tend to overhype higher framerates, especially those who claim there to be a big visual difference between say 120fps and 144fps. If I play at 60fps for a few hours and then drop to 30fps in a game, yes it will feel different at first, but then after playing for 10 mins or so I will be used to 30fps again.
Also I think the framerate importance is different on different games, for me driving games, I consider the most important for higher framerates, and also I think game menus should be 60fps, but otherwise for the most part I am ok with 30fps. Also I dont like it when I hear the phrase "upto 60fps", usually thats developer speak for they capped at 60fps but the game wont maintain it and have variable framerate, I always prefer a lower steady framerate to a higher variable one, even on driving games.