As I noted in my article Comparing LTO-6 to Scale-Out Storage for Long-Term Retention, in these situations tape is an ideal storage type. Data on tape can still be automatically scanned for durability and it certainly meets the cost-effectiveness requirements.

Right now our blog on the presence of tape at EMC World is seeing twice as much traffic as all the other EMC World related content. Why? Many of our readers are coming to the obvious conclusion that tape, despite the negative marketing, is still an optimal way to protect and archive their information.

... All the drivers that started the replace-tape-with-disk movement in the first place - reliability, performance, portability and off-site data movement - are now liabilities in a disk only strategy.

Tape allows for a clean sweep of data that simply doesn't need to be on any form of disk but still needs to be kept. The cost and capacity of tape makes these 'just in case' copies very affordable.