There is some truth to this. A couple years ago I bought three tickets from AirAsia. A week before the flight there was a volcanic eruption which canceled all flights at the destination. They issued me a credit voucher for future flights. Half the credit expired before I next needed to travel to an airport they service.

I still use them because sometimes they are $300 cheaper. And they don't play hardball with luggage weights, unlike some full fare airlines like British ("Your bag is 7.8kg, the limit is 7kg, if you want to carry it on you need to remove five pieces of clothing and put them on yourself").