Daniel, a weapon mounted light does not prevent you from using a handheld light. If I'm searching with my gun drawn, I'd rather concentrate on aiming one object (gun with mounted light) instead of two (gun in one hand, light in the other). Even if you use the commonly accepted lowlight firing techniques, the gun is pointed at whatever the light is pointed at.
Some leo agencies have been known to deny patrol officers the opportunity to carry weapon-mounted lights on their duty pistols for fear that they would use them for a flashlight. a gun with a light on it is not a flashlight, it's a gun. I have a hard time believing any responsible adult would confuse the two. Idiots would, but they need to be fired.