Computers don't fail often, but it does happen. Backup gauges don't hurt (other than being a little more gear to buy and carry) but the need for them really depends what kind of diving you're doing. If you're doing a couple shore dives locally and it's easy to go again next day or next weekend, you may not care so much about the tiny possibility of having to bag a dive. If you're out doing 2 dives on a local charter boat you will probably care somewhat more, and gauges can be useful as a backup if you plan for it. If you have flown thousands of miles with the idea of making many multilevel dives each day, not only will you care a lot, but that's a situation where you are taking much more advantage of the computer and gauges just don't make a very practical backup. (That describes most of my diving and I dive with a backup computer.)
(Hoseless air-integrated computers have the most ways to fail. Besides something happening to the computer, the link between the tank and computer can fail or hiccup. Many people think a backup analog SPG is a good idea if you're using one of those.)