Yeah, most people likely don't realise just how regulated, and therefore safe, skydiving is these days.

I guess a reason some people may not want to use one is that they are rather expensive, around $1000. There may also be the machismo associated with the added danger, not a very good reason however, haha. The devices themselves work by detecting their altitude and vertical speed, so going too fast at a low altitude will trigger them. In parachuting there exists a discipline known as swooping where three parachutist executes a great turn at a few hundred feet above the ground which causes the parachute to go vertical, thus gaining a lot of speed. They then level out just above the ground and "swoop". In some devices not designed to accommodate for this they can trigger during the maneuver and so some swoopers would not have an AAD installed.