User blog comment:Kygehn/Wuju buy a Yomuu's?/@comment-10958553-20131225063829

Your focus is in the wrong place.

Garen purchases the Black Cleaver because he has the means to shred hit the opponent multiple times, as does Riven.

Master Yi and Talon purchase Yomuu's because they prefer to deal massive damage all at once, not necessarily through multiple hits. While Master Yi will auto attack a few times most of his damage comes from Qing in and Static Shiv. By then, the opponent should be far past half health or dead.

The real way to look at this is, Black Cleaver is for those who deal damage at the end of a fight, and Yomuu's is for those who deal it at the beginning.