You're confounding consumer technology with professional tape archival solutions.

AFAIK, all DDS (what most people call "DAT") formats are forward-compatible. That is, a DDS tape from an older unit is going to work with a newer one.

I've never used QIC, Travan, or other tape formats, and don't plan on doing so. I do the math on my backups page -- once you're past about ten media units, SCSI DDS is worth the cost.

For a drive plus ten tapes, your cost per GB is about $3. This is about comperable with current disk drive costs. The difference is that your incremental cost per GB is $0.75. CDR compares (I'm using 650 MB and $0.50 per disk) at $0.76/GB. This neglects compression, but both units should give comperable performance.

The advantages for tape:
  • Low per-unit cost.
  • High reliability. Both media and units have a long track record of reliabile use. Media stores well.
  • Relative insensitivity of recording to stream buffering. Your tape writes aren't going to be trashed by a system which gets periodically pegged.
  • Reusability. The duty cycle on tapes is high -- DLT is rated at 1 million head passes, DDS is lower (and requires cleaning), but respectable. Media is cheap enough that replacing questionable media is a no-brainer.
  • Capacity. A 20GB tape provides 30 times the capacity of a CDROM. With single disk sizes blowing through the 100 GB threshold, a CDROM is to your hard drive what a floppy was to a 250 MB disk in the early 1990s. DVD helps the situation a bit (18 GB capacity), but still has the problems associated with creating backups -- a single glitch can blow the job.
Take care to note the distinctions between what are typically professional solutions -- DDS, DLT, AIT -- and consumer technology: QIC and Travan. In most cases, consumer technologies favor low aquisition cost (the drives are cheap) and high media costs. There's little premium on reliability or media compatibility. I have to agree with you regards removable storage. I still use floppies only. If I can't net it, it doesn't happen. CDR is the de-facto sneakernet standard these days (I'm not saying CDR doesn't have its uses). But as floppies were never really suited to backups, CDs, IMVAO, aren't either.

In the PC world there was also the additional hassle when dealing with tapes that you were always intermediated by some sort of archival software. There's a strong reason why I advocate use of simple, standard, tools, such as tar, over other tools.