There are several other cards that are great to get from the Black Market deck precisely because they are more valuable if only one is in play. Having the only Witch, Sea Hag, Mountebank, or Familiar ensures that curses won't run out. Having the only attack makes it less likely that players will buy defenses against it, and having the only defense or curse-trasher in an attack-heavy game makes it more likely that other players will buy (or already have bought) attacks and you will avoid some of them. Having the only Pirate Ship or Thief gives you more chances to hit because you won't be competing with others to eliminate coppers.