Frame mounted ones have to be really stout. I had one that I thought was strong but flexed enough with the full weight of the rig on it to dent the rocker. They also need a big enough gap to account for bushing flex.
I plan on welding on something similar to this on my factory rails to keep from caving in my doors. I heard it mentioned in one of the 1,000 videos out there that when running the Rubicon they made "changes on the fly" but adding more protection to the gas tank skid and something mentioned about the rails from some of the "damage" from the trails.
....... also, as Brad mention in the video posted in the above, it is common to use the front locker and the rail to "turn" or "maneuver" around rocks, by using the rail as a pivot point. Having these outer rails help from caving in the doors when doing these maneuvers.