Well, they worked pretty well against the bridge defense system in that same episode, at least to the point that it was able to buy Kirk a few crucial seconds to run the helm before the alien being killed him.
Is that what's going on there?
The sequence of events is this:
1) The alien takes control of some of the ship's systems
2) Kirk tells Spock to lock the helm console with a "static shield" to prevent tampering of key systems
3) This shield is put in place, without using any visible hardware
4) The alien is thwarted by the shield, and starts blackmailing our heroes, first by reminding them that it has the power to shut down life support; at this point, some of the heroes are wearing their life support belts (although not in all the shots!), probably because of the fact that the alien controls life support
5) The alien starts shooting at the heroes with the intruder control system; Kirk and Spock take hits while their belts are on (although the belts disappear in many an intervening shot!) and are outlined in the familiar forcefield yellow
6) Kirk crawls to the console, punches some buttons and removes his belt
7) Alien ceases fire and issues Kirk further commands, which Kirk obeys
It seems to me, then, that the belts did nothing to protect Kirk. The beams were effective in hurting him (which was the aim - there was no lethal intent) and forced him to accede to the enemy demands.
8) The alien gets the heroes to rig special controls so that it can completely take over the ship
9) Kirk convinces the alien that the controls require manual intervention; he presses the button for that
10) The ship is now controlled by Kirk, who initiates a suicide plunge
11) The alien resumes blackmailing attempts by firing at Kirk, who is surrounded by the glow of his belt (even though the belt still rests on the helm console where he left it at surrendering) and takes multiple hits while already standing next to the console and having completed his rebellious act
12) Kirk withstands the pain, and the alien loses the game of chicken and departs
Again, I don't see evidence of a real protective effect here. Kirk just refuses to surrender to nonlethal force a second time, since he now finds tactical advantage in resisting. But the first time around, he
wore the belt, and still had to surrender after just two shots. The second time around, he
isn't wearing the belt, and shrugs off half a dozen shots by a more desperate enemy.
I would venture that the stun setting on phasers is as widely used as it is because it can still be effective despite personal forcefields while more dangerous settings--disruption and thermal--are not.
I could agree with this, yes - but we still have no evidence that the more dangerous settings would have less penetrating power. We see nonlethal beams inflict the sort of damage the enemy hopes for, both in "Beyond" and "Slaver Weapon", and we see kinetic attacks thwarted, both in "Beyond" and "A Fistful of Datas". This combination is in full agreement with the idea that personal shields are only good against kinetic attack but cannot resist phasers on any setting, which explains why the belts are not combat gear outside TAS or even within TAS.
As for the stun setting, it is widely used by Starfleet, which has doctrinal reasons for nonlethal weapons. We don't see it used by the villain species much if at all. Individual civilians of villainous persuasion often use stun for criminal gain, but that's a different issue...
Timo Saloniemi