Adding red each time you cast a spell and not having that mana leave your mana pool as steps and phases end is sweet, and can lead to some really stormy lines of play
You may pay the squad cost any number of times
You can see that Terra has an ability called Trance, and Esper Terra is classified as an Enchantment CreatureSaga Wizard and has a sequence of abilities including her drastic transformation
An example is Summon: Shiva, which plays like a normal Saga, starting with an immediate effect and getting a lore counter each turn, before getting sacrificed after the last one is placed
The only black based decks which dont play Thoughtseize at all in Modern are highly synergistic decks like Goblins or Mill, or decks which cant afford to play it due to other restrictions like Living End
Of course, the two can overlap, but it makes it far more likely that youll try and build an aura-based draft deck around Uril, the Miststalker, even if there are better things to try and do