Just because the energy density is measured in a certain way, it doesn't then follow that the energy density when measured in a certain way is going to be significantly worse.

J/L is actually one of the more important ways to measure battery efficiency for transportation. In many discussions around this theme you'll find this to be used in comparison with a gas tank for size and that ultimately translates into range, arguably the most important feature of an electric car.

Higher energy density means higher range, all other things being equal, and with the weight of cars being what they already are a battery weighing a few hundred pounds would be perfectly acceptable.

So J/L is actually very helpful.