"Bare hand" tracking technologies such as the Microsoft Kinect or those from Leap Motion are performed in the most natural way: there is no need to hold a controller, wear a glove or use any accessory. There is no equipment to lose, no wires to untangle and no need to fit something to your hand size. Of course, your hands are also free to do other things - such as type on a keyboard - without having to put the controller down.
Controller-based hand tracking devices such as the Hydra from Razer currently offer more sophisticated control by allowing the user to operate pushbuttons, switches and analog sticks. One would expect that over time, you could get the same precision from "bare hand" trackers.
What you can't get from a bare hand tracker is the haptic feedback, such as vibration, tactile feedback or reactive grip. Is this feedback valuable enough to warrant holding a controller in your hand or will most users choose "bare hand" trackers?