Full suspension mountain bikes make lots of sense for lots of people. Rather than asking your body to absorb the worst of rough, rocky terrain, your bike's rear shock takes the abuse for you. If you have chronic back or neck aches, a full-suspension bike will allow you to ride further than you ever thought possible. Let's kill off a myth here.
Some people think full-suspension is just for going downhill.
You get the benefit of having extra traction for going uphill. The rear shock keeps your rear wheel on the ground, so rather than bouncing up rocky climbs, your rear wheel digs in and lets you spin to the top. You're controlling the ride, not merely surviving it. This basically means your seat, crankset and bottom bracket are on the main triangle of the frame. Whether you're seated or standing, you always keep your body weight on the frame, allowing the rear suspension to stay active at all times.