I really think there was a potential to do something extraordinary with the cards to put a fresh face on the game that was missed out. 1% per star seems like such a waste given the amount of coding required to add the cards to the game. Imagine instead if the cards had done something useful, like +stats, hp, block, discounts on mist potions, increased effectiveness of regen armor, raises to max level or stat caps. There could even have been sets for extra bonuses. A few points of free energy per day. With a little imagination, there could have been thousands of possibilities to make the game more interesting.
...and then there could have been random card packs for sale for a few gems..