*If* your fighter is using a sword-and-board, then it will occasionally be useful to attack a secondary target in hopes of splashing onto your main one with Cleave. Any 2-hander using fighter should have Reaping Strike, which largely invalidates the whole concept (unless the very high AC opponent is a minion).
The key is that with a 1-hander, you are given up less [W] damage (on hits) and at a 50% hit rate on the low-AC guy you are doing the same damage to your desired target as Reaping Strike would on a miss.
The situation will come up, but rarely enough that you might well only notice in retrospect. In average-damage calculations, be *certain* to include criticals. The tactic is only useful against very high AC opponents, and as such *most* of your hits would also be crits (go go removal of crit confirmation rolls...). Note finally that if you are in a situation where using this tactic makes sense, you should probably be spending more mental effort on Running Away.