User blog comment:ClariS/Myth on Reduction/Penetration Flip/@comment-4022742-20130304092530

For me it's not so much the penetration that is the problem. With the change to penetration they also made the change to resistances, making it more expensive to buy, they also buffed health items making them preferable over resistances.

Weather or not you notice the actual flip or not is beside the point, seeing as how aside from a few (Rammus, Malphite, Taric, Galio) not a lot of champs stack armor or MR.

This usually makes picking up TBC/LT a no-brainer and Last whisper only comes in play if the enemy has more then 120 armor anyways... (late game, if you even get there)