So far in 2020, we've been averaging about one per month (and if you go back to the initial release in December, we've actually had 13 individual drops in three months, making the average closer to four drops per month since the series was announced), although the average is somewhat complicated by how you count themes that come with multiple drops, like the original Theros Gods during Worlds in February (technically, the Gods were five individual but related drops, although there is an argument for counting them as one big drop since buying all of the drops together was an option)
Even when it kills the creatures
It slows down decks that try to cast multiple spells in one turn like Storm, Tron, or Cascade, and it also messes with greedy mana production
Watching them devolve into chaos over one card is a blast
That alone is nice, but the additional bonus of being able to save a single creature from a removal spell means the ceiling of this is high, it comes in untapped