And people continue to not understand how penetration works.
News flash: Buying Last Whisper doesn't read "+10,000% physical damage." If you're actually building to counter Zed, I.E. buying Armor, then no matter how much Armor Pen he builds your effective MR against him is going to be lower. Always. Without exception.
Want me to prove it with basic mathematics? Because I can prove it with basic mathematics.
Lets say Zed's rocking a Last Whisper. That's 35% Armor Penetration, right? Right. Now, lets say you're the average champion at level 18 with a base Armor of 75 and a base MR of 50 (these are rough estimations but serve to illustrate my point well enough.) Now, lets say you've got MR per level runes and flat Armor runes, bumping these numbers up to about 85 Armor and 70 MR, respectively. Now, lets say you aren't a moron and you're buying Armor to counter Zed, not MR. That means you've probably bought a Randuin's Omen (+70 Armor), Iceborn Gauntlet (+60 Armor), or maybe even a Frozen Heart (+100 Armor) So your total Armor is going to be around 155 or higher. Reasonable assessment, correct?
Armor = 155
Magic Resistance = 80
These numbers seem reasonable to you for the average bruiser? Okay, good. Now, lets apply 35% Armor Penetration to this equation.
Armor = 155 - (155 x 0.35 = 55) = 100
Magic Resistance = 80
My would you look at that. Even with a Last Whisper, if you haven't built any MR, your effective MR against Zed is going to be lower than your effective Armor.