[Original message removed]
Qi Coins work like the rest of the captive currencies in the game as you use them to buy unique, special, or difficult to obtain items, not convert them into g. So there's no reason to change Qi Coins while leaving the others as they are.
The odds at the Slot Machines are tilted enough towards the Farmer winning that over time it amounts to unlimited free Qi Coins. Which therefore would be free g which breaks game balance. The Spinning Wheel at the Fair also works this way, with odds stacked towards the Farmer so she can get more Stars but not to convert into g. The Qi Gems at Ginger work the same way, you can get as many of them as you want though there's no easy multiplication like at the Casino or Fair. Even the Golden Walnuts work this way, though those are a fixed total amount like Bitcoin and seems to have similar difficulty when trying to find the last one, dagnabbit!
Instead these minigames are all really about buying those otherwise unobtainable items, so play the games and enjoy the prizes. And fill Ginger and your Farm with Ancient Froot and Kegs for your super g profit dreams!