Dimir Control leans on Counterspell and Snuff Out to protect its board and answer your threats, so Duress taken before you commit your real payoffs (Troll of Khazad-dum, Pestilence) clears the way and can strip a card like Extract a Confession or Cast Down. The extra Nihil Spellbomb is a low-cost inclusion that can exile their graveyard to deny Deep Analysis's flashback. Cast Down is a weak swap since most of their creatures (Faerie Seer, Spellstutter Sprite, Augur of Bolas) are already small enough for your Pestilence activations to handle, so proactive disruption is more valuable than more removal.