In that particular context they are talking about power density, which is W/kg. Energy density is another issue, and I probably confused them above.

With gasoline in a modern car getting 50 MPG you can go over 500 miles without stopping. But so what? You still have to stop to relieve yourself once in a while--in my case about once an hour, say 100 miles. Increasing the range beyond 100 miles is decreasingly important. The EV-1 had a practical range in bad conditions of about 100 miles, and batteries continue to improve...