This is definitely true, and I probably should have noted we do use kerosene a lot still, but in my experience Juice per Stuff works best for explaining to people the limits on just about anything that moves and carries it's own fuel, including rockets, cars, and cell phones, keeps it standardized and lets you use the same concept for all, rather than introduce specific impulse, there's a nice intuitive connection with MJ/kg. The core point is that this stuff all functions on the same order of magnitude and that if you want big payloads on the cheap (relatively speaking) you can't do it with chemical rockets, energy/mass IMHO works better for emphasizing that, nice and linear and tangible, and showing it's relative value to food or explosives grounds people back to reality better, again IMHO.