I can see where you are coming from. New players do farm donation coupons, but that is very time consuming and there are other items that would take higher priority (armor, weapons, auras, etc.). That would be a lot of work to obtain those coupons for a .01% chance. Using donation coupons favors the people who already have enough to repeatedly use them on this or those who are able to donate to obtain more.
There are plenty of events that anyone can win like dice and warp portal. You are rewarded with 7 coupons each event. Requiring event coupons would encourage more players to participate in events.
I am not entirely for one or the other. Requiring a small amount of both could work nicely, as Xtopher said.