The actual reason a thinner (butted) spoke is mechanically superior is the amplitude of stress fluctuation in the spoke, not to mention the in the rim and hub at the spoke interface (as the wheel rotates under load) is reduced. This increases the number of rotations these components can statistically withstand before fatigue failure can be expected. Obviously the number of cycles to failure is different for the rim, spoke and hub, and is highly variable for each component as well due to the stochastic nature of metal fatigue.