I think your final statement is overly simplistic. There certainly are configurations where raid1 cannot save you - the classic being 2 PATA drives on the same channel - one master, one slave. In this situation you will live through simple media errors, but not much else.
On a good SATA controller, any failure on one drive - including firmware crashing - should not affect any or device for more than a few seconds, so your uptime guarantees will be much better.
md/raid1 is certainly not a complete solution in itself. But when combined with sensible hardware choices, it can make your availability significantly higher.
