The problem isn't really with Cleave, it's with AoO. They give you an extra action, and at base that doesn't make a ton of sense. You might quite reasonably ask "why does one guy dropping his guard mean that another gets whacked?" But step back. Why does that guy dropping his guard mean that he gets whacked in the first place? Now, I like AoO, and I don't mind ZOC rules elsewhere. They serve an important purpose. But the implementation is less than perfectly realistic.
It makes sense for one combatant to lunge at another when his guard is down, but he's going to get one full attack every 6 seconds no matter. There's no mechanic for testing an opponent's guard, and the feinting mechanic is wildly different from AoO, if they're meant to model the same thing. Nor for waiting until they present an opening while presenting none of your own.
Now, none of that is a bad thing; nonrealistic combat has been a hallmark of D&D for 30 years. But it does mean that the lack of realism will occasionally be highlighted, as in this situation.
For my part, I'd argue in favor of following the RAW, because 1) IME, players love it when they get to Cleave. Let 'em have a little satisfaction now and then, and 2) it draws attention to the tactical aspects of combat, which I think are fun.
-C.