Rebuilding a RAID array does degrade performance to a mildly noticeable degree...but restoring from backup - especially an image backup - completely annihilates it. Progress and performance are both exactly zero as you sit about twiddling you thumbs waiting a few hours to get on with your life. And that's only after you get back from the hard drive store (with the replacement) which will hopefully still be open at whatever ungodly hour the thing decides to go poof at.
If you live in an area that has really bad power, then you should have a UPS anyway. If you work on large and/or important projects...and really hate suprises...then you should have a UPS anyway. It's not specifically a RAID cost.
Single disk systems, still have disk controllers, and those controllers are also just as easily subject to failure. So that's not really an exclusive to RAID issue either. So if either type of controller gets just flaky enough...data corruption can occur. But then again if a stick of RAM gets just flake enough...data corruption can occur. Yet strangely nobody ever warns against (hay more sticks is more points of failure) installing too much memory...

Now granted RAID5 is a bit much for the average home user (even if I do like running it), but RAID1 mirroring is a great way to be sure your system will be on-line when you need it.
I've seen backups fail, I've seen RAID arrays fail ...(IMO it's best to do both)... RAID failures had a lower anguish rate however.