The ability that defines Duelist of the Mind's power works in all zones, not just the battlefield
The main aim is to basically outlast your opponent by having more removal than them, then using their own cards against them or playing Sheoldred, the Apocalypse and drawing your way to victory
Since Peer Past the Veil is still on the stack while it's resolving, it won't count itself
The second mode drains your opponents, and the third gives you card advantage for each opponent with one or fewer cards in hand, which is relevant if your deck is built around making your opponents discard their hand
Banned in the Five Color format
Even if the Curtains die to a Lightning Bolt or Unholy Heat (rarely with delirium on Turn 2) after blocking the Monkey, they still blocked the Monkey before trading, when the typical auto-loss pattern is having your Turn 1 creature die to removal on the spot while your opponent gets a free Treasure and card flip