Seems to me that a Red elaborated on this not too long ago: it's really very simple; armor pen (and % shred) is completely useless against true hit points. QuadCleaver also doesn't grant much defense, which means that even with less AD/AP than them, you should still win, because even if you have to attack one extra time per 3/4/5 of their attacks, they still have to hit you some 40 times before you die. And odds are they can't take 40 of your hits.

This is of course a very narrow scope here, and I was rather hyperbolic before trying to drive the point home, but the fact remains that thinking outside the box will allow you to prevail against cleaver stacking.