The first ingredient of the answer is that for all sorts of video (including games) to look smooth, it is important to know exactly when a frame is going to be displayed on the screen.
The second ingredient of the answer is that, with a fixed monitor refresh rate, you're basically forced to run at a divisor of that refresh rate.
So, if your game usually runs at 72Hz (on a 144Hz monitor) and you get to a busier section, the framerate has to drop down to 36Hz, even when 60Hz would still be possible in terms of CPU and GPU power.
I get the divisor thing (if you're not doing triple buffering), but I don't get why it should be divided by a power of two. After 72Hz it should be 48Hz (still very good) and only then 36Hz (still not bad I think).
Anyway, I would be curious to try to see if I could notice a difference. Math can take us just so far :)
The second ingredient of the answer is that, with a fixed monitor refresh rate, you're basically forced to run at a divisor of that refresh rate.
So, if your game usually runs at 72Hz (on a 144Hz monitor) and you get to a busier section, the framerate has to drop down to 36Hz, even when 60Hz would still be possible in terms of CPU and GPU power.