These threads are always interesting. I assumed this phenomenon was real because of how many times it comes up but interesting to see people say otherwise / there apparently being a lack of anything concrete explaining why.
I'm still having issues with the forum taking me to the wrong "new" post sometimes, so I skipped over this.
There's an easy explanation:
Most "30 FPS" options in games are (typically bad) frame rate limiters unless they are a V-Sync option, and most people trying to run games at 30 FPS use frame rate limiters - then they complain that it's not smooth and properly frame-paced.
When you have a 60Hz display, it
has to update every 16.67ms. If there is no new frame prepared in time, it will repeat the previous one.
When you want to display 30 FPS at 60Hz there are 60 "time slots" available that frames can be placed in.
Bad frame-pacing is when, rather than there being a new frame displayed every other refresh, those 30 frames are unevenly distributed across those 60 available refreshes.
Any frame-rate limiter has the potential to cause this issue, but some are better than others. RTSS tends to be the best option for a well-paced 30 FPS at 60Hz, but even so, it is not perfect - especially if you are using it on a system that is just barely achieving your target frame rate.
Half-Refresh V-Sync is different. Essentially what it does is treat your 60Hz display as though it is a 30Hz display.
So rather than there being 60 refreshes that those 30 frames may be distributed in, there are only 30 available to the game.
If the game is running at 30 FPS, you essentially
cannot have frame-pacing issues because there is only one way that 30 frames can be distributed across 30 refreshes.
The only reason to avoid using half-refresh V-Sync is if you care more about latency than smoothness/frame-pacing, or if you have an AMD GPU. Unfortunately AMD does not offer a half-refresh V-Sync override in the driver like NVIDIA does.
If your display supports a 30Hz input, selecting that and using V-Sync rather than 60Hz should have the same effect. I believe the "Special K" mod tool can also force a "Present Interval" of 2 on many games, which is the same thing as NVIDIA's half-refresh V-Sync option.