TFA explains why he didn't use RAID1: "I put in one hard disk per device for maximum redundancy: any hardware component can fail and I can just use the other device.". If the motherboard dies on one device, he still has a working device.

A backup to one computer is better than none.

A backup to two computers is better than one.

A backup to four computers in two geographic regions is better than two local.

There's no end to how good a person's backup can be. Everyone has their limit.