The reason for using the engine to brake is probably safety. If you just use the brake pads and its a long enough slope (more than an entrance ramp surely) you can heat up the brakes enough to damage them, warp the rotors, or worse yet as I've heard described on CarTalk, boil the brake fluid causing the brakes to fail completely.

All cars use the engine to slow down (or in our case the motor). In fact in manual transmissions you are taught to downshift when slowing down on a grade, NOT to just hold the clutch in and brake.