It draws a given number, in this case four for five mana, one more mana and card than Harmonize, Concentrate or Ambition's Cost
This type of deck loves a little self-mill to get it going, while also having plenty of ramp to be able to recast those cards
Burn is rarely completely gone from the Modern metagame, due to its power and linear nature
95% of the time, the triggered ability probably isnt worth a removal spell thats going to be better sent towards Rhystic Study or Smothering Tithe, but once enough cards have been exiled, the risk from rolling a 20 is going to weigh on your opponents
This being said, the expected value is probably better than you think, so if you are feeling a bit lucky, opening a box of Masters 25 isn't a wholly losing proposition from a financial perspective
Blue-Black Witch looks to keep the board in check, then use cards like Sedgemoor Witch to finish things off