Since Riot has made Armor Penetration generally more expensive, would it be a better idea for AD champs to have Armor Pen marks than Attack Damage marks? Then you could just buy more AD in-game instead of the pricey Armor pen?
The AD marks are for very early game only, even before the balance changes armor pen marks meant more damage fairly early in the game against champions. That AD is for early laning damage, especially against minions since it makes last hitting easier. If you are very good at last hitting and don't try to engage early on then armor pen is better for you. By the time you bought substantial AD items it wouldn't really matter anymore I think.