I think that would really be a bad move, people are watching videos on their phones, how would you do that on your wrist for a longer period? The more the device covers of your arm the more likely it is to get accidentally damaged. A phone can be used with either hand, a wrist strapped device can't quickly be passed from on to the other wrist. Using the camera to take pictures would be very difficult. As cool as it may look in a film/movie, in real life it just would be awkward.
