Dimir Midrange is a grindy control-ish deck full of counterspells (Counterspell, Force of Negation, Subtlety) and card advantage engines (Psychic Frog, Orcish Bowmasters), so we want efficient, flexible interaction over narrow tools. Consign to Memory stops their own Orcish Bowmasters and Psychic Frog triggered abilities plus any colorless permanents, while Mystical Dispute is a cheap answer to their Counterspell, Force of Negation, or Subtlety in counter wars. Orim's Chant is weak against a deck built to grind with instant-speed interaction and creatures, and Spell Snare overlaps awkwardly with our own Counterspell/Spell Snare package that's already well-positioned, so trimming a couple copies for more flexible answers is correct.