hong said:This is correct. Even if you take AoOs out of the equation, you can still end up in the situation where the BBEG takes more damage if he's accompanied by his minions, than if he's not. The problem is with Great Cleave itself; AoOs just make it obvious.
What I'd rule is that you can only make one cleave attack per target per round. So you can knock over as many mooks as you want, but you'll still only get one swing at the BBEG.
Actually, this is not correct. While Great Cleave could grant you far more attacks than normal, you would still get a maximum numberof attacks against the BBEG equal to your total number of attacks/round (barring of course AoO). The reason for this is that presumably the cleaving attack sequence will end when you hit the BBEG. So the problem is not at all with Great Cleave, but with AoO and cleaving in general.
Moreover, I forget who said that because this was a "should" debate, it belongs in the house rules forum. I move to disagree. This is precisely a rules argument because in this case, the rules make no sense - or at least in the spirit of the rules, allowing cleave to work unqualified with AoO makes no sense. As it has been stated, restated, and restated again, it simply does not make sense that a BBEG with mooks would be easier to defeat than a BBEG without mooks. It goes against the entire CR and EL system (even as defunct as that system is). For this reason it is a rules argument. As a player, sure, I'd love to see a broken tactic such as allowing cleaves to be used with AoO especially since it would rarely be able to be used against me (I know, irony at its best). However, as a DM, I would never allow this. The bucket of snails, or bag of rats shows how this mechanic is a slippery slope leading to nonsense. As with most slippery slope antagonists, there is always the argument to have the DM just arbitrarily block the descent. The problem with this is that it is arbitrary because the reason it is a slippery slope in the first place is that it is founded on a nonsensical premise - one that contradicts the CR/EL system.
The clearist way to make cleaving with AoO consistent with all of the other rules and with the CR system is to rule as an early poster suggested that you may not cleave into another foe with an AoO unqualified. That qualification can be any number of things. It could be that you could only cleave into another who also provokes an AoO. It could be that the provocation of that AoO has to occur during the same initiative count, etc. You could even rule that the cleave works on the same target that provoked the AoO, kind of like that demon ability (possibly devil, IDHMBWM) that allows a cleave attempt against the fallen opponent virtually guaranteeing death.