AOO is set to All Enemies by default. Oh wait. I'm in a battle against Ragzlin and like a dozen cultists. On Lae'zel's turn, I switch AOO to only trigger on Ragzlin because he's the only person I want her to AOO against of every enemy near Lae'zel. From that moment on in that battle, Ragzlin is the only target she will AOO until I switch the preset again.
Yeah, and my counter-example of why I wouldn't want that is this: I set my AoO to work only on Ragzlin because I consider him the priority target. During his turn Ragzilin does absolutely nothing to activate it, let's say he even moves first and just attacks on the spot. I'm left for the entirety of the remaining round wasting all my AoO opportunities while two other goblins could have triggered it.
So... I wasted more time that confirming a prompt, I had to make a decision upfront that involved switching toggles, selecting targets and what else; in the end it amounted to absolutely nothing. Why should I be happy with this system, again?
It's more like this. Lae'zel goes. You set Ragzlin as target. 4 enemies go who don't trigger AOO because Ragzlin is target. With prompts, each 1 would trigger a pause prompt. But regardless, you'd skip these 4 anyway if you prompt controlled it because you are waiting for Ragzlin to go to see if you are going to get AOO on him.
Next round. Ragzlin turn. He doesn't trigger AOO. He just stands and fights. Several cultists go after. Then Lae. 1 moves that would trigger AOO, but you set Ragzlin as target. Oh well. You maybe missed 1 AOO opportunity before your next turn.
OR ... You know those several cultists come after Ragzlin in order. So, you set Ragzlin AND all those cultists as targets. Then, if Ragzlin doesn't trigger, you might have 1 of those several trigger it and not miss your AOO. Still same result but no pause prompt.