Stacking it on Proving Grounds is a bit silly.

still only allows 4 stacks....

Yay, I reached to 4 in half the time?

The armor pen is actually worth stacking... So basically, if you rely on physical skillshots, stacking them is useful (you don't get punished as hard for missing 1)

Because stacking a purely offensive item leaves you vulnerable to damage and CC?

i saw a zed stack 3 black cleavers, a BT, GA, and Merc Treads.

he had 3k hp, 150 arm and mr, and could do 2.5k burst damage.

Items kinda rigged.

Pantheon with 2 Black Cleavers, a GA, and a Frozen Mallet or something. It was ridiculous. The stats on BC are good enough to warrant stacking it, so it really does not need any more incentive.

Ok. You guys don't understand this concept. youmuus does the same thing. Instead of stacking two BC's, get a bc and a youmuus. Then you don't have a wasted passive/active, and the stats are nearly worth the same. You lose 20 damage and 250 health but gain 15% crit chance and 5 armor pen.