Have you ever pulled it off
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
At this point its basically a bulk rare
(Though, if you have enough mana fixing, you could maybe splash for a third color if you wanted.) But if you want to shop around to other colors (what's more Dimir than having a Dimir deck box when you're actually playing Selesnya, I suppose?), things that may draw you into specific colors are: A really strong rare you're excited about
Do Unstable basic lands and Amonkhet Invocations (both cards where the art goes to the edge of the card) have borders for the purpose of this card
Yes, in fact, they make up the bulk of these boosters