It’s not just about how smooth the game looks, but also how smooth it feels to control. 30 fps is way too sluggish for me. Granted, most people would probably reach a point of diminishing return somewhere after 60 fps, unless you’re someone with the reflexes and hardware (high polling rate mouse, good frame timing on your monitor, low system lag, etc.) to back it up. I’m quite comfy between 120 to 144 fps, but there’s some absolute monsters out there who would probably find that too slow.
If it’s not a very fast moving game, like a turn based RPG, then it doesn’t matter that much, but at least 60 fps is still a must for me to not look like a slideshow.