Stopping your opponents from casting their commanders from the command zone is the tip of iceberg: Nothing is cast from exile, from graveyards, or from libraries, as this Drannith Magistrate shuts down casting from every zone except your hand
Forcing your opponent to block the equipped creature is a pretty rare effect, and can enable some interesting synergies
("Why are people against it?") I said they were a cross between transforming double-faced cards, which the majority of the players loved, and split cards, which the majority of the players loved
Keep drawing cards so you always have something to play
Portent of Calamity For five mana I can drop Omniscience if its in my top four cards on top of drawing three cards
In an environment where you start with 40 life, paying 2 to come in untapped is far less meaningful, so these shocklands should be considered "enters play untapped" by default