For a database, or DR, I do agree backup to disc is fine. You typically only need the latest version with the most (and most up to date) data.
For backing up disparate documents, however; xls, docs, pdfs, etc.. you wind up with a ton of different versions of the same file, often changed daily, which can be corrupted at any time. (Interestingly, where I work, our users often don't seem to notice this however for weeks..!) I see this from time to time. Even with dedupe, if you have enough versions of all these files and have terabytes - petabytes of data to backup, a totally disc solution would be very pricey as the capacity requirements skyrocket. Tape also has the benefit of being extremely portable, as well as long life. (Granted, speed is a disadvantage).
We're strictly tape at the moment, but we are moving towards a hybrid solution shortly, incorporating into our environment either Data Domain or Quantum's DXi.