User blog comment:Myself v1.0/HP over defenses? O rly?/@comment-2248060-20130124230024/@comment-216.36.26.248-20130125061954

It's relatively easy to build Black Cleaver and Last Whisper. Doing so gives you 54.5% armor penetration (after a couple hits). Meaning you're getting less than 50% value out of any armor purchased.

Buying HP counters this. It also counters magic damage by virtue of AP characters tending to be burst damage rather than sustained.