So, for any given Limited Card, each team could include 1 copy in either Duelist's Deck, making a total of 1 copy that team can use
Each Kethis activation lets you replay your Opals from the graveyard, while Jace mills yourself to fuel even more activations
The Mirror Breaker has fallen off in Modern and other competitive formats, but its still a favorite in Commander decks
However, these cards are occasionally self-sufficient, and Melmetal ex is an intriguing option for players who like to set up the board throughout a slower game
I would probably template the Blade-licker demon to care about permanents going to the graveyard instead of being destroyed because having to track whether something that dies was destroyed or not seems painful, and also a lot of layers are not terribly aware of counts as "destroying" and what doesn't