Really? Do they publish data on how many vouchers (or value thereof) they've issued and how many of them are actually used? Besides, if the voucher convinces someone to fly again who otherwise wouldn't (or convinces someone to choose JetBlue who otherwise might not have), it still probably is a profitable endeavor from a marketing perspective. Remember that pretty much every voucher also means real money coming in, as they aren't often issued in values that cover entire trips.

ps- Site is still down.