Dimir Midrange is loaded with blue and black interaction (Counterspell, Thoughtseize, Fatal Push, Force of Negation, Spell Snare) plus Orcish Bowmasters and Psychic Frog as efficient threats, making Veil of Summer excellent to protect your key spells, draw a card off their removal/discard, and gain hexproof from blue and black. Tormod's Crypt strips their graveyard to deny Psychic Frog's flying activation. Both come in for the situational Sewer-veillance Cam and one Boomerang Basics, since bounce is weak against a deck that can simply recast cheap threats and counter your follow-up.