Chains of Mephistopheles is very good vs. BUG Shardless because it hits Brainstorm, Jace, and Ancestral Visions. It makes Ponder and G.Probe worse in Storm or Tempo matches too. The only place I really want it though is against Jace, since Pox has a hard time with planeswalkers. I don't think it's crucial, just an effective option. Use Pithing Needle vs. Jace until you win the lottery. I was about to spring for some, then TNN made Jace worse, then Reid Duke made the price go up, and now I'm having second thoughts. I play Chalice main, which helps against Brainstorm decks, so my evaluation of Chains' importance is potentially skewed. Chalice also bolsters my Storm matchup, so, again, potentially skewed perspective.