Edit: It's not about making the "best" decisions. It's about having Full Control over your characters, also considering time required to give a certain level of control.
I guess it is because ALL your exemples (in which you always know what's coming next) attempt to prove that you could make a better choices with pos-up You'd probably have to change a few things at every combats, obviously (mostly targets). But don't forget that most of the examples given in the previous pages are all about the wizards and eventually the sorcerer...
It's true that you don't *know* what's coming next, but you can certainly guess. And since you'll already have information about at least one enemy did (because they're currently prompting a reaction pop-up from you), you can make a more informed choice than you could on your turn. Obviously you can still make a mistake or fail a gamble; you could not use your reaction because you think the next enemy will cast fireball, but then they cast a cantrip. But this decision process is more on the player and less on the whims of Larian AI, again because you have more information at the time of decision.
