Perhaps my example should have been more clear.
Mr. Pike (that's what we'll call him), has moved to within 15' of a line of say, 10 guys. Mr. Pike attacks one guy, kills him, great cleaves a few of the other enemies, then takes the 5' backwards, leaving the remaining enemies 15' to 20' away, within reach of his pike. One of the enemies closes, leaving the 20' threat square into the 15' threat square, and provokes an aoo doing so. Mr. Pike then takes the aoo on the closing opponent, then proceeds to great cleave those enemies still within his range, enemies who did nothing themselves to provoke any such attack, other than to be friends with a stupid npc.
I agree that the example sounds pretty ridiculous, but it was even more ridiculous at the table. I wish I had a picture of the battle, as my description may be a bit difficult to follow. Unfortunately, I was only a player in this game, and my opinion that such a maneuver was ludicrous, was vetoed at the table.
I know this wasn't what the OP probably had in mind specifically, but it illustrates the point that there are feats within the game, cleave/great cleave among them, that can be abused in certain circumstances.