Dimir Midrange leans on a heavy removal and disruption suite (Bitter Triumph, Requiting Hex, We Say Thee Nay!, Nowhere to Run) so protecting your threats matters more than pump spells that just get two-for-one'd. Secret Identity and Shore Up give your key creatures hexproof to dodge removal like Bitter Triumph and Requiting Hex, though note Nowhere to Run specifically lets them target your creatures as though they didn't have hexproof, so it still gets through. Spell Pierce punishes their proactive noncreature plays like Kaito, Bane of Nightmares. Cut the weaker pump effects that don't survive their removal-heavy game plan.