As funny as it sounds, building more armor can counter penetration, and health too if the target isn't a strong BORK user.
Why?
the flat armor pen items. Pretty expensive. Unless you're a glass cannon champion, warrior and just brutalizer are expensive compared to
that negates the difference. As for last whisper, your armor is still useful it's not just as useful. As annoying as that may be, it helps keeps tanks from being unkillable late-game.
Though with the recent armor/health item nerfs, you'd think they would give LW 5 more AD and reduce the armor pen to 30%, but that's just me.
You know what annoys me more tho? I main support/jungle, but even I'm annoyed by this one thing in midlane:
vs
. 100 gold difference, but that 100 gold gives merc a 10 MR advantage and a really useful passive. So if an AD assassin is vs a mage...
Not to mention sorc boots is countered by
. Not as bad (Aegis is a very nice item!), it just sucks when the AD assassin built merc treads. :/