This is all fine and well, but this is not taking the game into account.
The job of the ADC is to kill front line tanks, fighter, and bruisers. 200 Armor is nothing. If a fat cho with like 4K HP and 300 armor is running at you, 2 PD's and an IE is going to do nothing, even with crits. A LW would absolutely shred his armor though and let you easily kill.
Another thing to take into account is sometimes you are only able to auto attack once or twice before repositioning, LW makes you hit harder against front liners which are likely trying to get to you and kill you.
This is why I go IE - PD - LW - GA.. It's a much better build for the job of ADC, which again is to get rid of the front line, while AP nukes the back. Of course you have to think for yourself in every situation, generally you are sitting there attacking the closest target because if you chase into the fight, you die.