Don't know about you, but I do just fine with zero hands on the wheel. On a straight stretch of highway, with good alignment, I can drive for miles without ever touching the steering wheel. In fact, I'm pretty sure I could drive the same highway while asleep.
But, people should keep both hands on the wheel (as much as possible) for the obvious reason:
You can't react as fast or as well. If something unexpected happens, having both hands on the wheel and being conscience both greatly increase your ability to perform emergency maneuvers.
There are very good reasons to take one hand off the steering wheel: turn signals, transmission, using any of the car's features that are meant to be used while driving such as cruise control buttons. They all increase your risk of accident, but are all necessary in some way. Just driving around with one hand permanently off the steering wheel for no good reason is irresponsible.