My problem with this is that if the enemy does start building armor. Your BT components are worth roughly either a Thornmail, or components of Randuin or FH. The only BT component that builds toward LW is Long Sword. It's potentially a lot of delay. I've had this situation screw me several times, especially when the enemy manages to catch up in the process. Suddenly, I'm not doing any damage to their frontliners.
This is one beef of mine with the current tank items. Penetration became too important when Riot started adding truckloads of health into the top armor/MR items. Back in S2, Randuin and Visage didn't have as much health. There was Aegis but it was more of a mid-tier item.