Dimir Midrange is loaded with countermagic — Counterspell, Force of Negation, Spell Snare — so Exquisite Firecraft's spell mastery clause making it uncounterable once there are two or more instant and/or sorcery cards in your graveyard is a huge upgrade over less resilient burn spells. Skullcrack gives extra instant-speed reach to help close out the game through interaction. Bump in the Night and Skewer the Critics are fine spells, but the uncounterable text of Exquisite Firecraft and instant-speed flexibility of Skullcrack are more valuable against this much interaction. This matchup is a grind where you need your burn spells to actually resolve, so prioritize resilience over raw efficiency.