Some countries offer a choice of a full cash refund or an additional amount if taken as a travel credit eg.120% of the fare in the form of a travel voucher which is perfectly reasonable. Future travel credit may be worthless to someone who booked a domestic flight during a holiday in a country he won't be returning to, on an airline which doesn't serve any routes he can use. Also if the ticket was purchased by someone who travels very little, for a particular event such as a job interview or wedding, then credit is also little use.
A voucher would be beneficial to someone who travels a lot on that particular airline and would welcome an extra 20% value when making his next booking.
Either way, it should be the customers choice as to the form of refund when the promised service isn't delivered.