Scenario: A customer requires 17A4 compliance.
Solution: I build a 17A4 compliant solution by configuring CMOD properly with the correct hardware.
But seriously, the scenario depends heavily on what software / hardware and budget are available. A lot of people are moving off of tape because of the ongoing maintenance costs, and the ever-decreasing cost of disk. The only thing that makes disk more expensive than tape nowadays is electricity -- keeping those disks spinning 24x7x365.
Tape is still an excellent solution if you can properly define the 'cutoff' where data isn't being accessed anymore (*ahem* RAPTOR4) -- because tape doesn't consume electricity when it's sitting on the shelf.
But all in all, a 17A4 compliant solution probably includes a bunch of 'locking' disk, specially configured to lock-on-write the data loaded into IBM CMOD, WORM tape backend, and TSM/SSAM providing access to it from Content Manager OnDemand.