Their use of this developer setting also triggered an obscure and intermittent bug reloading icons which created inconsistent results in their lab
That would obviously chew up battery life downloading icons over and over again, chewing up CPU cycles to refresh the icon on the window, and chew up wifi power because its easy to see how a stuck loop re-downloading icons could cycle thousands or millions of times.
Good for Consumer Reports for sticking to their guns, seeing an issue, reporting the issue, and forcing Apple to fix it. It's obscure sure. But many developers probably use Safari Developer Mode to work on their projects, and this will help them.