Ah OK I get it, I haven't had the chance to get out into open water yet, but itching like mad to get out there.
Has anyone ever actually had a dive computer fail on them. I would imagine they are pretty much fail safe, unless you forget to change the battery
Someone here had a Shearwater go black on them mid-dive recently. I wasn't following the thread to know what ever came of it, or if the cause was determined. Computers are susceptible to failure by their nature. There are a lot of ways they can fail. Fortunately, failures are so rare they probably border on statistically insignificant.
If/when your diving reaches a point that your life truly depends on the data a computer provides, redundancy becomes mandatory. If you're doing no-stop/no-decompression/minimum decompression/whatever you want to call it "recreational" open water diving, you really don't
need to worry about redundancy. I definitely won't say it's a bad idea, and I don't think anyone would ever try to talk you out of having redundancy if it makes you feel better. Just be aware that it truly isn't necessary for diving within "recreational open water" limits. IF your computer fails (which is extremely unlikely) you can simply terminate the dive and make a safe ascent to the surface at any point. You should be proficient enough to do a safe ascent without instruments guiding your ascent rate, and you
should pay enough attention to your instruments to have a pretty good idea "where you're at," in the event everything blinks out on you. You'll become familiar with your gas consumption rates, so if you check your gauges regularly and suddenly your air integrated computer is a blank screen, you'll easily be able to think "a few minutes ago I had 1,800PSI, so I know I should be comfortably over 1,500PSI now." That's
not an excuse to continue the dive, because you "know" you're over your planned minimum gas, but it should be enough to recognize you have more than enough air to make a safe, controlled ascent, and hold a safety stop when you get there.
You'll also most likely be diving with a buddy, who can check depth and time as well as any analog backup can.
If you feel better with redundancy, by all means. Just don't feel any pressure to spend money on it.