As we know that every boss in game (Quest boss,Instance boss and Field boss) have chance to drop card. However, Not every class need to use card. Some class like sorcerer or necromancer is main class that have skill to summon boss out from card while many other class use it for card battle mini game only. Which make card have no value for them at all when got one from field boss.
It would be good if card can trade. Because boss card will need ‘card’ to level up star. As we can see that now ability of card will increases by 10% per card star. If it’s trade able, any class that need a card can buy from other player who don’t have any use with card. Which mean card that get from field boss won’t turn into something that’s completely useless for them.