Oh stop raining on the fantasy parade, keep your facts and rules of physics to yourself! ;)

I think the primary energy source has to be gasoline or diesel. Energy content and logistics.

Is there something I am missing? Have there been studies that walking on two legs is the best thing for robots? Or is that just because it's a feat of engineering to get the balancing act perfect?

The robot mule was four legged.

I think the idea is that a legged machine can get through really difficult terrain that even a tracked nachine cannot.

In one video a marine says that the robot can get through about 80% of the terrain that the men can.

Edit: apparently walking robots can also be very efficient, if they walk in a way that has the robot "falling forward" like most legged animals do. I suppose on smooth terrain a wheel is the most efficient way to move. But on rough surfaces, a legged creature can step over high and low spots and adjust its leg length to, in effect, convert uneven terrain to smooth terrain.

I think the research is worthwhile, but the fact is, a drone simply dispenses with the need to navigate terrain.

I think it's worthwhile too. Ten years ago few people imagined the current efficiency of Li-Ion batteries or solar panels. If we sit and do nothing until all the problems are solved we'll never solve all the problems entailed in designing something like this. I can see these things being useful for hauling heavy loads in challenging terrain.

I think the military is working on or has small drones that a squad can hand launch for reconnaissance. Can't carry the troops' loads though.

Check out this little robot, it moves easily over terrain that, relative to its size, is incredibly rough. No tracked robot of the same size could handle this terrain.

Boston Dynamics: Dedicated to the Science and Art of How Things Move.