UB Reanimator is also a glass-cannon combo deck trying to cheat Griselbrand or Atraxa, Grand Unifier into play via Reanimate, Animate Dead, or Shallow Grave; Thoughtseize lets us proactively take away their reanimation spell or a Dark Ritual before they can go off, which is more reliable than Cabal Therapy's single guessed name. Cabal Therapy also forces us to sacrifice a creature, a resource we need for our own combo. Destroy the Evidence does nothing against a deck not leaning on a fragile manabase for its main plan. This is a straight race, so keep our full combo density and use Thoughtseize purely as insurance against their faster draws.