That's actually a reasonable argument if you have breeders. The U and Th in an average continental crustal rock will give that rock 20x the energy output of burning the same mass of coal.

My understanding (and I am a layman) is that lithium isn't that accessibly common on Earth, and that we're doing a bang-up job of using as much of it as we can get our hands on already. Are there practical methods of extracting less accessible lithium that don't themselves have nasty side effects?