We have two independent computers, and a Garmin Etrex Vista. But what do we use? Well the stoker computer has had a dead battery in the sending unit since the fall, and she's saying it doesn't matter. The Garmin is for navigation on tour, and stays home otherwise. The primary use of the one remaining device is the odometer, although I occasionally look at the speed to know at what speed we stopped pedalling on a down hill, and therefore at what speed to re-start in the current gear.
Having the odometer allows me to track the mileage on tires and chains, which I care about to know how about when they're likely to need replacement when we're on a long enough tour that they will need replacement on the road.