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. "

Travel Troubleshooter: Vouchers rarely worthwhile [Star-Tribune]