The trucks didn’t run on electricity.
How useful would an electric truck be, with today’s technologies and costs?
I have no idea what you are talking about. You seem to be arguing tha mules are better than trucks, I would say you need to stop drinking.
Big Dog can run on a variety of energy sources including gasoline, fuel cells, diesel, hybrid systems with gas/battery, etc. Currently it runs on compressed air in the lab and gasoline in the field.