Aux fuel has other benefits. For a basic cert ride, no, it's not necessary at all. For endurance rally use, it's very helpful to be able to get gas when you want to instead of when you need to. After that, in touring mode, it eliminates some fuel stress when you're in Alaska, Eastern Oregon or other wide open places out West. You can just get gas when it's convenient, rather than having to start looking for gas at 200 miles, (or what ever your benchmark is).
One of my HD friends did several cert rides, SS1K and BB1500 levels, on the tiny stock tank on her HD Softail Slim, but she was very clear that no way she would try a BBG on that bike w/o aux fuel. Just too many gas stops. Every bike is different. The Feej has a big tank and good range. And mpg doesn't drop like a rock over 80 mph either.