Being a former mainframe IT consultant, I can readily affirm the necessity of not only HAVING a regular backup schedule, but also having restore capability immediately at hand, if needed. One regional bank I worked at kept a continuous log of all online activity/updating, 7-day cycle of 8 hour major-file backups on site, and kept duplicate 24 hour backups, one on site, the other, about 20 miles away in a secure facility. Losing track of someones' or some companys' money is NOT an option.
For the professional photographer, a regular backup schedule and methodology is also necessary. Hard drives fail. Some more than others. And as noted above, they frequently fail s-l-o-w-l-y. So a simple backup A and backup B methodology would likely both have the same damaged files. But then, one must ask, why is 'forever' storage necessary? It's not likely you'll ever need to retrieve photos shot 10 years ago and print more pictures. To that end, I'd probably move any shoots that are more than a year old to duplicate DVDs stored in separate locations.
For the bulk of us non-professional photogs, I suspect anything more than a weekly backup of what your 'current' work is overkill. Although hard drives are now less than a dollar per gigabyte (vs $250 for 30 Megabyte drive 25 yrs ago!) and RAID may seem an easy solution, from what I've read, RAID is not-so-easy for a computer novice to deal with, especially in the event of a failure.
Through all my years in computers, one rule has always remained...KISS (Keep It Simple, Stupid). And that's how I still do it. My post processing methodology is still evolving, but here's what I do at present:
1. As 2nd or 3rd level protection from accidentally wiping out a picture or 5 while doing post processing, I do not delete/erase the memory card until ALL processing and delivery is completed on that shoot. Memory cards are cheap insurance.
2. I also shoot in RAW + JPG, and transfer both sets to to physically different drives my computer.
3. I then work on only one set of RAW in Lightroom, and output as JPG to a different folder. When all my LR work is done, I copy that output folder to another, and use the copy as input for Photoshop editing. Some things are easier in PS than LR, so I use both, as needed. Photoshop outputs replace the files they came from, as I still have the Lightroom output 'originals' in another folder.
4. When all is complete, I make a copy of the 'finished product' to another folder, on another drive.
5. I print, make CDs or DVDs, or PowerPoint presentations from there.
6. When everything is done, I move the RAW, LR output folder, and finished products to a long-term 1TB drive, deleting everything else on my 'in process' drives. If it's a shoot I really have to keep 'forever', I may burn a DVD or two of the images.
7. The 1TB drive gets backed up to clone 1TB drive when convenient...hopefully, monthly, and keep it in my file cabinet at my office.
8. To facilitate making drive clones, I have each of my drives in 'hot swap' bays. I simply turn off the computer, slide in the backup target drive, make a backup, and remove it. SATA-3 transfers are so much faster than USB 2 or even 3 to an external drive.
In the event of incoming major weather problems, fire, riot, or anything else, I can slide all the drives out of all my computers in less than 30 seconds and be gone! And, oh yes, ALL original purchased software CDs as well as downloaded software (purchased and free-ware) are copied on my long-term hard drive...as is a WORD document with -ALL- serial numbers, etc. If I have to start over from nothing more than my HDs, I can.