I suspect that the engineer in the GP's story might not remember the equations for angular acceleration, but has an intuitive sense that if a spinning something is in contact with a non-spinning something, you need a good reason for why some of the energy won't get transferred from the former to the latter.
In theoretical model of ideal XXX. Not when asked to actually calculate something practical unless the ideal model is good enough for constraints.
I am not an engineer. Actually MS in physics however ancient ;)