It's true that there are so few places in the lower 48 where fuel isn't readily available that carrying jerry cans or Rotopax for fuel on a Jeep is more of a fashion statement than a necessary accessory, but judging by the number of Jeeps at events like Easter Jeep Safari that carry cans or Rotopax it's an extremely popular fashion accessory, which to me doesn't make it wrong.
There are places in North America though that fuel isn't readily available and carrying extra is a very good idea. Alaska, for example, the last gas before Prudhoe Bay at the Arctic Ocean on the north end of the Dalton Highway is 250 miles south at Coldfoot Camp. That's just about the comfortable limit of my LJ depending on speed and other factors, so when I did that trip I carried extra fuel.
The Dalton is also very well known for tire punctures, lots of bolts and assorted crap fall of the trucks on that road so extra spares are also a very good idea. I carried extra fuel and spares on the Dalton, and I'm planning more extended expeditions like that, so better ways to carry extra fuel and spares does interest me.
And carrying extra fuel on that Alaska trip gave me the flexibility to explore off the main road, like this side trip to the Koyukuk River.
I'm planning exploring northern Canada and extra fuel will be a good idea there too. And occasionally on trips off road in Colorado I carry a 5 liter "just in case" extra supply of gas. A 5 liter can is pretty small and can be 25 miles or so of extra range so it can be nice to have when the gauge approaches "E".