Duress strips Counterspell, Snuff Out, or a removal spell before you commit your threats, which is huge against a deck built to grind you out with permission and card advantage. Faerie Macabre adds extra graveyard disruption against Ninja of the Deep Hours lines and generally thins removal targets elsewhere; it's a reasonable inclusion though not essential since Dimir has few true graveyard synergies. Tithing Blade's sacrifice-a-creature mode is weak against a deck full of card draw and few vulnerable permanents worth forcing away, so it comes out. Bojuka Bog is low-impact here since Dimir doesn't lean on its own graveyard, making it a fine cut for more proactive disruption.