Can someone explain why armor penetration runes are good? The maximum penetration you can get with runes is somewhere around 25 right? Doesn't that equate to +25 physical damage dealt? That doesn't seem very impressive.
With 25 Armor Penetration your damage modifier would be 100 / 175, or ~57%. You would do 57 damage.
This looks even less impressive than +25 physical damage (which would be amazing), until you consider that Attack Damage marks give +.32 damage per mark, versus +1.96 armor penetration per mark.
Using OP's Xin Zhao as an example, Xin's base damage at level 1 is 55.3. Garen's base level 1 armor is 21.7. With no armor penetration Xin would do 45.4 damage to Garen, but with full armor penetration marks he does 53.1, or 7.7 extra damage per hit. Full attack damage marks are 2.88 damage.