But it's not really an exploit either. Say the BBEG has an AC of 28 and 300 hit points. Now, say he has 4 mooks, each with an AC of 14 and 10 hit points. Now take the big-bad-nasty-fighter PC. Rather than look at it as thought the fighter is trying to hit AC 14, look at as the fighter trying to hit AC 28. It just so happens that he's bad enough at his current level that merely being in the way of his swing could kill the mook.
I really think this is a matter of perspective. The "this is wrong" camp views the fighter getting extra attacks by attacking the mooks. I view the fighter simply attacking the tough guy like normal, but it just so happens that the mooks, who are wimpy enough to drop in one hit and aren't that hard to hit in the first place, are in his way.
If the mooks don't represent a real challenge anyway, meaning if they can be easily used to Cleave off of, then what's the problem? After all, if that's the case, then their presence has no effect on the EL of the encounter to begin with. In effect, a fighter that is cleaving off of the mooks and hitting the BBEG with each cleave is simply proving that the presence of the mooks make little to no difference in the overall difficulty of the encounter. In other words, he's proving that they might as well not be there.
