Izzet Spellementals wants to chain cheap spells like Opt, Sleight of Hand, and Prismari Charm to reduce the cost of Hearth Elemental and Eddymurk Crab, so Spell Pierce is excellent at taxing their early tempo plays or countering a key Traumatic Critique. Secret Identity and Shore Up protect your creatures from their bounce (Prismari Charm) and removal (Burst Lightning) while keeping the clock on. Since they have minimal creature-based removal, we trim the redundant Octopus Form copies and slower pump spells in favor of proactive protection and disruption.

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.

Dimir Excruciator's plan revolves around discard (Duress), removal (Requiting Hex, Bitter Triumph), and a grindy Doomsday Excruciator/Day of Black Sun late game, so keeping your threats alive with Secret Identity and Shore Up is key to closing the game before they stabilize. Spell Pierce punishes their cheap disruption spells when they're tapped out on turns 2-3. We trim slower pump effects that don't help you race their removal-heavy plan.