Using sugar for human consumption is, in a sense, quite wasteful.

I claim, that given sugar, using it for human consumption is one of the least wasteful things to do with it.

This is still inefficient compared to other means of using the same sunlight.

In the future, if there is an option between powering organic people with sugarcane-produced sugar and powering cybernetic people with solar cells, and we can choose to be either organic or cybernetic, then your argument will be valid—assuming there are no other options, which is silly. For right now, people need food. Converting sunlight into other forms of energy in other ways is fine and good, but personally, I would also like to keep growing food for me and my brethren.

though again this is only sugar-to-mechanical efficiency.

This is a big caveat. A typical person burns much more energy maintaining homeostasis than they do in moving. Following that, brain activity is the second-largest energy sink. While athletes can quadruple their caloric requirements (indicating that mechanical energy can become the largest drain on energy), I think calculating energy conversion with your example is suspect.