sigh
Yet again, more evidence of players demanding that their random strangers [NPCs], 'Be more trustworthy!
"Un-fun, un-fun——respect the chosen one!—
ME!".
Thanks for the link to a (forgotten) blast from the past posting.
(I was able to catch some spelling & quote box errors, and restore a missing animation.)
There is nothing unreasonable with the concepts suggested in that post, and they have been used by the best in the industry.
I wasn't sure what Drool meant, or in what context it was meant; but now that I've seen it—probably (unless that's not it), I do stand by it wholeheartedly; and the RPGs that omit, or skirt around this [for non-technical reasons], are ultimately a disappointment for it; because they could have done better.
____
When the player hires a mercenary, that person (or creature) is not one of their PCs; it may be a very helpful party member, but they are their own entity operating on their own agenda. This
should mean that the player cannot force them to act against their will—they are not totally beholden to the player; they might want to help as a staunch ally, but they might instead want to rob the party blind.
In other cases, they might simply not wish to travel in some places... like underground, or in cemeteries; or to be present when the player attacks their own guild—exposing them as a traitor, or instead choosing to warn their guild of the planned attack... or simply not wishing to attack their other comrades. (Too many RPGs allow the new-hire to turn about and murder their old village. Only the most villainous characters should agree to do that. Player Actions should always be filtered through the NPCs agenda to determine if they will go along with it.)
Consider the situation with Bombata, in Conan 2.

He joins the party along with his charge, as escort. He [begrudgingly] obeys Conan, and proves a very helpful addition—initially, but he is trustworthy only insofar as it serves his own agenda. Once he has what the party had set out for, he tries to kill, or otherwise trap/delay/bury Conan, and everyone else. He then returns the prize to his own city, along with his charge intact.
In game terms, this would play out as a controllable NPC that watches the situation, and decides if they will still accept commands from the player. This was done in Baldur's Gate, Baldur's Gate 2, Planescape, Temple of Elemental Evil*, Realms of Arkania, Eye of the Beholder 2*, Menzoberranzan, etc... This avoids puppet NPCs, and adds credibility to their personality. They don't all have to be dishonest. Doubtless, some will share the PC's agenda, or general outlook on life, and be fine with their actions most of the time; but consider an NPC priest whose party defiles—or destroys graves, or the priest's own holy symbols, or robs a church. What do
you think that the priest NPC should do? Do you not think they would revise their opinion, and degree of esteem for their [...suddenly former?] companions?
In the Baldur's Gate series, the NPCs watched the player's reputation, and might abandoned or attack them if pushed too far; they more commonly had grievances with certain other NPCs, and would eventually pick fights with them.
There was a silly oversight however, where the player could use one of a pair of NPCs to kill the other; these pair are companions who travel together, who join—and depart from the party as a group. One of these pairs is a married couple, another pair are two irascible miscreants. It doesn't make sense that the player could decide (
and have) that they wanted the wife and not the husband (or vice versa), then command one to help kill the other.