Seeing a lot of youmous and b.c in the bot lane.

Really it's Caster ADC's who benefit the most from this. BC triggers off any Physical damage, and any amount of armor pen, is going to benefit an AD caster, where AS and Crit chance don't.

Lucian. in particular can easily benefit from BC since his Light slinger and culling can easily stack it. And Jihn tends to build ghost blade, since he doesn't scale normally with AS and crit chance. But those are the only LCS ADC's who buy those items, where others tend to have very different build paths.