I think the important thing here is that the term "backup" only makes sense within the context of what kind of thing you're protecting against.
RAID is a useful backup for drive failures, if you're able to replace failed drives within a certain amount of time, but not for manual deletions. DVDs and tapes are useful backups for manual deletions, but not for fires. DVDs and tapes, stored at an offsite location, are useful backups for manual deletions and fires, but possibly not for earthquakes.
Since all of the different backup and archival methods have major pros and cons, the only way to say something meaningful about how useful they are is to talk about them within context.
We've solved this semantic issue. Redundancy is used to refer to availability while backup is used to refer to recovery from data loss. Hence the R in RAID.
RAID is a useful backup for drive failures, if you're able to replace failed drives within a certain amount of time, but not for manual deletions. DVDs and tapes are useful backups for manual deletions, but not for fires. DVDs and tapes, stored at an offsite location, are useful backups for manual deletions and fires, but possibly not for earthquakes.
Since all of the different backup and archival methods have major pros and cons, the only way to say something meaningful about how useful they are is to talk about them within context.