Dimir Midrange is loaded with countermagic (Spell Pierce, Spell Snare, We Say Thee Nay!) and reactive removal (Bitter Triumph, Requiting Hex), so Duress proactively strips those answers before we commit our threats. Gollum the Abandoned provides a recursive threat that punishes their removal-heavy plan. Dissection Practice is too marginal against a deck full of efficient removal and card advantage engines like Enduring Curiosity, and Dark Knight's Greatsword is clunky against a deck that can just Bitter Triumph the equipped creature.