Thinking About Attacks of Opportunity

I get Attacks of Opportunity. I do, though I also think they are ridiculous. The rational part of my brain says “yeah if Olak the orc decides to flee after I walked up to him and gave him a good thwomping I should be able to attack him.” The old schooler in me says “But you already moved this turn when you walked up and thwomped Olak. You can’t move again until next turn” The books describe AofO as reactions Almost as if they are an automatic move, without much thought. OK I see, but if it’s auto without much thought then maybe it’s not as effective as a planned attack. I’m a well trained fighter. When I walked up to Olak I sized him up, I checked out his weapon and anticipated how he would use it, I saw the bits of his armor that I could penetrate. These quick thoughts came in and I used them to thwomp him. It worked, I broke through his defenses and gave him what for. Olak splits because he doesn’t want anymore. I can AofO him but I gotta do it fast. It’s a reaction, I just go in for a quick stab. I am not thinking about where to place the blow for max damage. I’m just thinking hit him! So why should this attack be just as effective as the one where I plan and thought through where to hit Olak? I don’t think it should be.

Or look at it this way Olak and I are engaged in combat. I attack him with a full thought out attack and missed. He decides to stay and fight and attacks me. I’m trying to block him with weapon/shield/armor, etc. My mind is now on how to avoid damage. Meanwhile Urggk  behind me just got thwomped by my Cleric friend Eldyn. Urggk now wants to flee. It’s crowded here due to close walls on one side and hot lava on the other. Urggk has no choice but to run right past me for escape. He does so I get an AoO. But I’m not focusing on Urggk. I’m trying not to get hit by Olak. So I can’t closely see Urggk’s defenses, armor, weapon etc, all the things a fighter would look at to exploit. No, I just see him and look to hit. So I do. Quick reaction hit. Again, I don’t think this should be as effective as a well-planned attack. 

EDIT: It's possible that the second example wouldn't provoke an Attack since I would be engaged in melee with another opponent. Though maybe it would. I'm not super clear on all the different ways AoO are provoked. (Old School Thinking and all.)