Only four aspect ally get piercing (at this moment we waiting for Age of Apocalypse box). Black Knight is second best (only behind Wolverine).

I thing Black Knight is still very solid option to consider in any leadership deck. If you compare him to Marvel Boy (one cost cheaper agression ally).. you got potentially 3 piercing for 4 resources. Which can be very good. But usually we use his ability just once (when he jump into play, or on the second point of damage). Outside Aggresion this is only viable option to constantly pierce. Plus Black Knight is Avenger, which can be crucial to put him to play cheaper.

Final Rating: (A-c) A in scenario with many toughs B in the rest of games