I am actually interested in the eventuality of caffeine (which they did mention) causing adrenal depletion. Those findings would apply even more violently to a more violent stimulant such as amphetamine.
I agree with their idea that one can get a spurt of willpower (and brainpower in general, in my experience) from a stimulant but the crash can be debilitating.
And no matter how much caffeine you have, unless you're taking it intravenously, you're spiking and crashing constantly throughout the day.