FYI: The concept behind the hollow bar is lighter weight, pure and simple. The stiffness is based on the diameter, whether it's hollow or solid (also affected by the material, of course), but all things considered, you can make the bar lighter by making it hollow, even if the diameter increases slightly.
By the time you get up to 1-1/8" diameter (typical front bar), as a solid piece it's pretty darn hefty to be hanging off the very front of the chassis (not where you want it for weight distribution, or polar moment of inertia).