And what exactly possessed Riot to think this was a good idea? This didn't work the first time, since it made armor useless to even bother purchasing, and you already have champs that not only reduce flat armor, but also percentile armor. Why is this going to work the second time if it didn't work the first time?
Sure, I can hit this.
Armor Penetration Pre-Season 1 looked liked this:
1. You could start with 40+ armor penetration. (Recall, we nerfed flat armor penetration runes)
2. Last Whisper was overpowered. (+40% AS, +20 AD)
3. Green Elixir was overpowered (Gave roughly 50% more statistics.)
The problem with armor penetration in this setup was that flat armor penetration was the de-facto standard for your rune page as a carry, but Last Whisper was also the highest damage multiplier after your infinity edge, and Green Elixir could fill in for the critical strike.
This led to carry damage turning on at roughly 17 minutes with full armor penetration.
In today's world - look at the differences:
1. Green Elixir is weaker.
2. Flat armor penetration runes are weaker than before.
3. Last Whisper isn't triple multiplicative statistics, only double.
It's much harder to full-spec armor penetration *and* have all your other multiplicative statistics at once. Basically, we changed the armor penetration formula to hurt carries - but the actual problem with carries was the ease of their build up.