User blog comment:Emptylord/Archived/PBE Patch Notes (Preseason 3)/@comment-23.30.226.209-20121128211240/@comment-5045559-20121129113631

Tested just now: It makes debuff stack faster. Also, while flat pen and %CDR are Unique on Brutalizer, they're not on Black Cleaver, i checked champion info window for every stat.

So yeah, unless they change that again, stacking Cleavers is now a choice (i'd get 2 max personally).