Ask him if he has taken into account the fact that the pavement is not level, but roads are designed with a crown in the middle so that rain runs to the gutters. As a result of that, a bicycle with no panniers, going straight on a level road could expect to see a greater load on the left hand side of the tire. To even this out, you would want to add a loaded pannier to the right hand side of the bike.

Don't forget to experiment on a frictionless roadbed with no winds...

95% of the time I ride with one pannier, on the non drive side of the bike. I place it there because it is big and red, and if a motorist is using that as their visual indication of my lateral position, I'd rather have them think that I am further to the left than I am to the right. Once you add in the effects of riding on a real road, the fact that there are turns, potholes etc any effect of non symmetric loading is balanced out - if not, I would see more tire wear on one side of my tires than the other - but I don't.