The loudness can be a factor; my computer at work is silent other than the HDs which I can hear, but isn’t annoying.
The ones at home I can’t hear over the fans in the case; the 120mm in the Turiq CPU cooler is louder than the drives.
As for if one of the 2 fails you being screwed… yea; can be. But that’s why I would suggest doing a system setup to the way you want it; then do an image to a back-up HD/storage HD (non-raid). I’ve got a separate 250G EIDE drive that I use for this as well as my archive of stuff; I’ll probably grab a 500G SATA or pair of 500G SATA’s for this eventually.