Originally Posted by Regory
Most champions simply don't bother with the quints because Sorcs + MPen reds give you roughly enough penetration to remove the base 30 MR most champions have.
Well, in the past 5 games I've looked at opposing MR values for the team and I think I only saw 1 person sit at 30 past 5m. Most even start with around 40-50.
The masses sat at 40-60 MR up until mid game, where the tanks settled around 120-150 MR with the carries still being between 50-80 MR in the late game. Some tanky DPS would hit 100.
While it hurts to overshoot mpen, most champions running minimal resistances won't be letting themselves get caught anyway.
With you being about to obtain 21 mpen from runes now, 20 from sorc, 20 from guise, going full flat mpen will make your damage very close to true damage against all opponents save for hard tanks. And even against them, you have Void Staff level.
Better than AP/level? The consensus seems yes.
What about CDR?
I promise you will be happier if you get cdr instead.
I'm not so sure about this.
Keep in mind, generally everyone builds armor heavier and much earlier than MR, thinking magic users' damage is lackluster in the beginning. This is the way the meta has developed. You're essentially countering this mentality by stacking mpen early, sending your damage through the roof vs. these targets.