Lemme tell you a little story about a game I played last night.
I was Leona against a full AD team. I had Targon, Sightstone, Sunfire, Randuins. Enemy Master Yi had Botrk, Youmuus, Warrior enchant, and Last Whisper.
He jumped on me with his ult and W up and used Youmuus and Bork active. I had my own W up and used my Q to stun him then tried to ult.
He had 1/2 of my HP gone before my stun landed on him, and then he killed me before my ult landed.
To be fair, my reaction time on ult could have been faster, but I did not anticipate being killed in exactly 1.5 seconds by someone who I had been specifically counter building.
Tanks buy stats specifically to mitigate damage. They give up damage to gain mitigation.
ADCs do not give up damage to ignore the mitigated stats of a tank. Using a LW against a tank with two armor items, that item is over 700% gold efficient.
TL;DR: THE DEDICATED ARMOR PENETRATION ITEM SHOULD NOT BE 100% GOLD EFFICIENT AGAINST SOMEONE BUILDING NO ARMOR.