The DEXA is useful because it's not just body fat %, it's muscle mass.

If you are on a training program, it's very helpful to know how much fat mass you are losing as well as how much muscle mass you are gaining. In my case over 3 months I lost 24.4 pounds of fat mass and gained 6.7lb of muscle mass. I knew I was losing weight by checking the scale, but I didn't know if I was just losing fat. Was I gaining muscle or not?

In addition the DEXA told me exactly where I was gaining the muscle...breaking it down between upper body, torso and legs.