Why, you might ask, do you really think two hard disks are going to crash at the same moment?
No, I don't. But mirroring doesn't protect us against files being corrupted by faulty applications. Or accidentally deleted. Thanks to Jeff Mitchell for the reminder.
BTW: What IS the probability of two hard disks in a RAID array failing? Two hard disks from the same lot? With the exact same load through all their lifetime? Within the time frame that it takes to rebuild the RAID?