To understand the danger, you need to look at the forces exerted on a handlebar. Most one-piece tubular handlebars, such as those used on virtually all cruisers, mount to clamps fastened to the top triple clamp with a single bolt each. With the handlebar removed, these clamps can pivot. As the rider puts pressure on the ends of the handlebar under braking and acceleration, the mounts still try to pivot, bending the handlebar in its middle very slightly. If the clamps mount in rubber, there is even more latitude for movement. This causes a minute flexing in the center of the bar. With a hole drilled in the middle of the bar, the bending forces concentrate there, in time causing cracks to develop. These soon turn into a fracture. One day you hit the brakes and both handlebar ends move forward, pivoting around the mounts.