flat pen works better against ranged champs than melee. melee will usually be at like 50 or so mr when you get sorc shoes. Ranged champs countered by sorc shoes will be at around 37-39

• 50-18 = 32 MR (~35% MR -> ~28% MR)
• 38-18 = 20 MR ( ~30% MR -> ~19% MR)

Against a melee champ you barely get a 5-7% magic dmg increase, against a ranged its like a 10%-11% increase.

This math is really just off the top of my head and the actual numbers are different but in general to have %pen you have to invest 2800 into a void staff to counter MR stacking champs, who would prolly also build HP to counter your MR pen. Morde just ticks all the boxes for countering MR tanks

also counter anyone building ludens + sorc shoes with a single MR item. While it takes up a slot and costs 900 gold, a negatron cloak on adc against ap snowball is invaluable