If you're using Disc-Disc-Tape backups, your staging disc area needs to be FAST. I've found (the hard way) that disc systems aren't anywhere near as fast as they claim to be.

If you're doing snapshots, then backing up the snapshots to tape, you'll be hammering the live discs (and hurting performance) whilst you backup the snapshot.

Most tape backup operations require the cycling of the tapes off site in case there is a disaster. The biggest problems for backups is the OS and how fast it can supply and take the data. Snapshot images of the disks get around some of these limitations but often will only restore to the system from which they were taken. True backups will restore to a different system which is all you'll have after a fire or flood. I'm not saying disk backups are bad, they just have different limitations. A good archival system would have both, disk for near-line and tape for off-line (off site) backup.