People are missing the target on the combo. Killing 30 kobolds is not the problem. That is just great cleave in action, allowing a fighter to act like afireball in limited situations.
The problem is getting extra attacks on the BBEG at full AB because his minions provoke and AoO. The problem is a midlevel character (4th level fighter, 6th level human, 9th level other raced) can generate multiple extra strikes at their full attack bonus on a BBEG because he has minions and they provoke an AoO. The problem is that these 'set up' AoO/cleaves are more powerful than the character's nomal attacks.
This leads to ridiculous contrivances just to create the situation. Instead of killing the minion orcs, you cast sleep on them and drag them to the BBEG's lair so that you can wake them up, watch them stand up or crawl away (provoking an AoO), kill them and cleave into the BBEG for extra attacks.
There is something there that can be exploited. If nobody will ever exploite it in your game, you don't need to worry about the problem. Go live a happy life.
If somebody ever decides that they will exploite it in your game, if you wait until they've built up a character over 9 levels specifically to exploite this isuue before outlawing it, that player will resent that you're 'nerfing' his character concept after he spent dozens of hours playing the character up to that point. If, however, you make a house rule that prevents the abuse of the combo before anyone starts to build a character that exploites it, then nobody gets hurt.
Cleaving off an AoO doesn't really make sense, anyways.
BBEG: "Gee, my minion let his guard down. I guess that means I deserve a solid thwacking when you PCs hack it up. I'll let you attack me when you kill my minion."
PCs: "Right decent of you, mate."