The reason our economy is coupon based is because the creators of the server decided it that way, and it's not a bad thing. It's easy for anyone willing to put in a few hours to make millions of zeny, hence if our currency was only zeny, we'd either have ridiculous zeny prices on uber items, or everyone would have them, and that destroys the point of rare and hard-to-obtain items.
Also, coupons can be sold for zeny, and the current economy for them is 20 million. So it's basically saying that if you pay 15 coupons for an MVP cards, you just paid 300 million zeny for it.