Flat armor penetration is a VERY undervalued stat in this game.
30 flat armor penetration is worth the same gold as 10 AD, yet you get WAY more damage from 30 arpen.
The reason is that brutalizer is a really cheap early game item for the damage it gives (arpen efficiency is taken from this item).
But you can't really get a lot of flat arpen in this game. Only two items build from brutalizer.
On a side note, if you have full AD marks, you get 8,5 AD, arpen parks give you 11,5 arpen.
8,5 AD = 306g
11,5 arpen = 138g
you can see AD marks are worth more gold, but you will still deal way more damage on most AD champions with the arpen runes once you get to lvl ~6 or so, when you have enough AD to make it worth, or your abilities have enough base damage.
Just because it's cheaper, it doesn't mean it's worse. But yeah, I would gladly get a warrior buff, (or a new brutalizer path), but we can't have that because of one guy that would abuse it every single game.
Fucking 
TL;DR gold efficiency =/= actual damage efficiency