It's a land slot, not a spell slot
This makes it incredibly strong for land loops in cEDH, often combined with many powerful self-sacrificing lands like Barbarian Ring or combo pieces like Hall of the Bandit Lord and Hermit Druid with Underworld Breach
While they are exceptionally specific in their abilities, they often occupy slots in a sideboard for certain matchups which would be exceptionally poor otherwise
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Turn over turn, Weeping Angel will suffer from the drawback of the opponent being able to turn it off as a blocker by casting creature spells, but this wont always be an option, and through discard or counterspells, there are tools for Dimir decks to save Weeping Angel from opposing removal, all while being able to attack the opponent or their planeswalkers with a vigilant creature that is quite unpleasant to block