AoO are supposed to simulate people dropping their guard, being distracted, etc., and their opponent taking advantage of that opening.
Uhm... I don't think that's the original meaning. That is more of an afterthought explanation of them...
I just think AoO were designed for tactical reasons:
- to prevent archers to shoot from close as well as from range (which could lead players to question why being a melee fighter if you can fight also in melee with a bow?)
- to prevent spellcasters to cast from up close
- to prevent running away from a fight
- to prevent passing by a first line of defense and reach the back lines with a simple move action
AoOs didn't
completely prevent those but provided a good reason not to try, which is fine since they're not supposed to be impossible things, just things that if you allow them freely they may end up being too good tactics, so they needed a price to pay to do them.
Unfortunately there were not-so-nice side effects of this design... one of them being exactly that as soon as they tried to "explain" them in term of "dropping your guard", this opened up a can of worms because everyone could claim the rules were st00pid by not allowing AoOs against characters who clearly drop their guard even more (like, someone sleeping!). One of the worst consequences
for me was the 3.5 update that allowed an AoO against someone standing up from prone. Realistic? Probably, after you've spread the "dropped your guard" explanation. Needed in the game? Not at all.
Then of course also the mechanics lead to some headache, not in basic scenarios but rather in cases where AoOs interacted with special abilities (Cleave) or unusual attack actions ("chains of AoOs"). For me it really sucked... it's one of those cases where the original
purpose of a rule is forgotten, the rules stop serving the game and become an excuse for some type of gamers to demand that everyone else at the table serve the RAW.