According to Christopher Elliot, ombudsman for National Geographic Traveler, they might not be:
“Vouchers are a better deal for the airline than they are for passengers. The carrier gets to continue overbooking its flights — which is common industry practice — and then offering compensation that is of questionable value to passengers.
There is probably a fairer way of handling an overbooking situation. I think airlines should offer credit that doesn’t expire and has real cash value. ”
We agree with Christopher, a voucher is nice, but it’s not really valuable. Vouchers shouldn’t expire, and they should be easy to redeem.—MEGHANN MARCO
Travel Troubleshooter: Vouchers rarely worthwhile [Star-Tribune]