I don't think the Sony developers thought this through. When you hold a typical controller, it is held in place by your fingers, but also between your hands. You can relax either part of your grip, and the controller is still held effectively in-place. This allows you to adjust the use of your hand muscles over time while gaming. It's a bit like getting up and stretching once in a while when you sit at a desk all day.
When you take away half of the controller, only your curled fingers are holding the remaining half. There can be no compressive force between your hands because each part is only held by one hand. Relax your fingers, even for a moment, and the controller-half can slip to a different orientation, or slide right out of your hand entirely. Now imagine that the controller-half is moving around rapidly, such as in a simulated karate game. Because the acceleration forces on the device are that much higher, you have to hold it that much tighter with your fingers to avoid slipping. After an hour of that, I think you'd have a hard time gripping anything with your hands. And over time, this just seems like it will end up in something far worse than carpal-tunnel syndrome.