Wow how much time was wasted on my example instead of the real problem that was posed? I guess I'll have to spell it out a bit more simply...
Forget the example, my only point was this:
Why does aiming for a secondary target give you an automatic chance to strike the first? Especially since it may be harder to aim for the primary target. This is what doesn't make sense to me. It's harder to hit the main NPC so I'll just aim for the minion is what I have a problem with. It could be that the NPC is low on HP's so all the players with cleave and auto damaging powers start aiming for minions instead of the NPC, with their main aim to take the NPC down. That's what doesn't make sense to me and that's what I don't like.
No, I don't expect D&D to be believable and be scientific, but like any fantasy or science fiction it just needs to be 'believable'. 'Just because' doesn't cut it. 'Because it's easier this way' has an argument going for it, but when it start messes with basic common sense it gets harder to cover up. Remember that D&D is based on telling a story, at least for most players, and telling a believable story is important. Combat is still part of that overall story.
Don't get distracted on anything above, we could start threads all over the place with that can of beans, just keep in mind my main point: Aiming at a weaker creature so you can hit the stronger seems silly to me and doesn't seem to fit.
Yes, for the vast majority of combat situations, it won't make much difference. But in some odd situations, I can see this situation as problematic and even game breaking if overlooked.
Take this to extremes. You're level 30.
You have maxed out strength to it's extreme. That's 30 strength for the people at home. (+10 str mod) You've got +6 from weapon focus, +2 from some other feat, +6 from its enhancement, and what the hell, let's say the DM gave you a bonus +10 damage because of some quest you did and he really likes you and your mom makes tasty cookies that he enjoys.
That's +34 damage bonus, on top of the 4d6 damage you do from the cleave power.
And you're up against a red dragon, and he hates you. He hates you so hard he brought in some minions, LOTS of minions. A bag of rats amount of the little rotters.
Now, you're up against this thing with over a thousand hit points... and about 16 rotter-bag-of-rats types.... and for some reason the dragon's AC is incredibly high. Some Deus-ex-mechina, perhaps. A McGuffin. It doesn't matter.
So you decide the best bet to damage this dragon is to attack minions and cleave into it with your mighty +6 weapon, with +20 damage from awesomeness, and +6 damage from coolness, and +2 damage from some feat I didn't even bother to remember the name of....
You hit a minion. You do... MASSIVE... damage. The minion has 1 hp, dies, no surprise there.
Then the cleave hits the dragon....
....
...for 10 damage. Out of approx 1.5 THOUSAND hit points, your game breaking plan does....
10 damage.
But my +6 to damage from my weapon?
Only applies to damage rolls.
And my +6 to damage from my feat?
Only applies to damage rolls.
And the +10 awesomeness bonus for the cookies?
Only to damage rolls.
And the +2 of feat-that-cannot-be-named?
Only to damage rolls.
So... altho you have all these damage buffs... because you're not rolling dice for damage, you're not bumping up your rolls with your bonuses, and if your bonuses aren't applying, this 'sick' 'exploitable' damage never actually happens.
Oh, and the dragon?
He eats you.
And burns you with fire.
And then eats a minion to be evil.
And burns you again.