I’d say it depends on your circumstances and your tolerance to the possibility of data loss. The general answer to the question is that without using some kind of redundancy, either mirrored disks or RAID, the failure of a single disk would mean you lose your data. This is true for each copy of your data that you have.
Off-site backup is the proper answer to your question. All this really depends on your own tolerance or comfort with the possibility of losing data. The rule of thumb is that there should be at least three different copies of your data, each in a different physical location. For each of them, there should be redundancy of some kind implemented to guard against hardware failure. Redundancy is typically achieved by using mirrored drives or by using RAID of some kind. Also, if you’d like to know, using RAID in which you can only lose one disk in the array is not typically considered a sufficient level of protection because of the possibility of a cascading drive failure during replacement of a failed disk. It should be at least two.
The calculations necessary to rebuild a failed drive from parity data stored on the other drives means that for the duration of the time that the array is being rebuilt (aka “resilvered”), you’ll have high activity on the other drives. So during that time there’s an increased chance that a drive that was already on the brink of failure is pushed over the edge. If that happens, your data is gone. Like I said it depends on your risk tolerance. You may not feel like it’s worth it in your situation. I personally only run a raidz1. I accept the risk that entails, just as people who use raidz2 accept the increased risk that entails over raidz3. There’s no limit to the amount of redundancy you can add. The level of redundancy that’s needed is a decision that only you/your organization can make.