The stats tell us that top pitchers do not give up more runs/hits/walks when they're comfortably ahead than at other times. Lets assume they do let up, that they do take something off their effort level. This implies that letting up has done nothing to their efficacy as a pitcher. Why wouldn't they then just pitch at this lower effort level all the time, if it hurts less and saves your arm?
BTW, as someone who does compete in athletics at an ehhh, fairly decent level and is heavily invested in things like nutrition, physiology and biology, I've learned one thing: Never trust the top athletes. The number of bafflingly-stupid things I've heard from top cyclists or runners or triathletes or swimmers about basic physiology and training is staggering. And those athletes are way smarter than idiotic baseball players who slam Red Bulls, still drink Gatorade, and wouldn't have a shot in hell of guessing what periodization is.