For just a TC there is no good reason to get a diesel, as you have noted it actually reduced the mathematical payload of the truck.
I also use my dually for other truck purposes like towing equipment trailers loaded with excavators around so as a dual purpose vehicle diesel makes sense.
One reason many people have a TC is they can tow large trailers behind them. I did that with my boat behind the TC also. With my fully loaded truck, camper, and boat I was around 18,000 pounds.
Also, a diesel dually is much easier to find on a dealer's lot and much easier to sell than a gas model.
If you are only hauling the TC or towing small trailers and you intend to keep the truck a long time I would get a gas engine.