When counter building for AD Metallo's is good on WW Prime but not on GasJoker. That leaves me two items for Attack armor,Booster's and Suit of Sorrows. Maybe if I am feeling trolly I could take Claw of Horus or Neron's. Both are not suited for a enforcer's role though and I would avoid both 9 times out of 10.
Now all of this is mitigated by the fact that on my enforcers I stack armor with Phantom's but the point is if I need more attack armor only having two, maybe three if your champion uses will, really viable options for counter building just isn't fun.