The question is not if a hard drive will fail but rather when. Hard drives have a finite life which is often times hard to predict; one reason why backup is so important. However, hardware failure is not the only threat to your data. Data can be damaged or held hostage by viruses that leverage modern encryption, disgruntled employees, accidental deletion or modification, or plain and simple corruption.
Backup plans should be comprehensive. They should take into account what data is most important, backup frequency (how much data are you willing to loose?), how long to store backups, and how quickly data can be restored in the event of a failure. For sensitive data, backups should be both physically secure and encrypted. Another aspect to consider is backup failure. What if your backup plan fails to execute? What if your backup media gets dropped or fails? What if your computer system and backup are stolen or there is a fire? A comprehensive backup plan does not rely solely on one method of backup. An on-line backup might be a good supplement and fail safe to your local backup system. But with any backup system routine checks and restore testing are critical.