It's more about data safety with the spares there while you order a replacement. If you're planning on doing backups and rotating disks it will take the pressure off redundancy. But in reality most people don't bother with a backup. It's more money and time... Laziness is a factor too I having to grab the backup disk and run the backup and verify it.reliability
Once you setup the raid you have a better understanding of how to keep it running and the maintenance side of it. There are utilities / scheduled tasks you can setup to monitor things. Obvious things would be using power protection to prevent a surge from wiping out the components. Keeping things cooled for longevity.