Yes, Delta should refund passengers for mass canceled flights. Corporations have an obligation to provide the service agreed upon with the customer. At the very least, it makes sense to provide the option of a refund. Many customers, of course, would be be fine with having the flight rescheduled. The point of good customer service is to make the offer of the refund.
Delta should refund passengers for mass canceled flights. This is because it would be irresponsible not to do so. There is a lot of competition between airlines and people can always move to the next brand if Delta isn't fair to passengers. It is extremely hard to get a full refund after a seat is purchased, so more power to the consumer.
Passengers affected by the mass cancelation of flights should be refunded, partially or fully, depending how much difficulty they were put through. Folks may have had connecting flights with other airlines that they missed and cannot get refunds on. They may have missed events or incurred additional expenses for travel or lodging. These passengers are not to blame and shouldn't be left to carry the whole financial burden. They didn't get the service they paid for.
Delta should take responsibility for the snafus whether it was caused by a power outage or not. The company should have had a backup power source in place, such as a generator, just in case something like a power outage happened. If these systems are vitally important to Delta's flights, then the company should have had safeguards in place. This situation proves how easy it is for something bad to happen if infrastructure doesn't live up to standards.