The other use I can think of is that Karn's card draw isn't worded as "drawing," so if you're a horrible person and want to lose your friends, you can pair Karn with mean cards like Possessed Portal or Omen Machine to lock them out of the game while you keep your hand nice and full
2024-03-08 If a player or permanent has more than one kind of counter on it, and you choose for it to get additional counters, it must get one of each kind of counter it already has
Yes, in fact, they make up the bulk of these boosters
Bristly Bill does a great job circumventing one of the issues with counter doublers that need another source to add counters in the first place
Polukranos, World Eater Polukranos, World Eater saw play mainly as a 5/5 for 4 with an upside, and anything with two colored mana symbols is fair game in the green devotion decks