Fore me the interest of trading card is for not pay for new upgraded card(but downgrading is also a good idea for thats) and for exchange rare card (for me is more fun than farm top 50 and not make useless hem because you still need rare for have other rare.
Also i have an idea , why not have (in-game) an event's organiser for all , = for exemple , a contest (battle , farming chalenge, etc...) and the player thats win the battle have one or many of your card(your chose the card(s)) or a certain amount of electrum coins.(of the organizer)