As a point of clarification, when Kodo Sawaki said, "Zazen is good for nothing," he was not implying that we shouldn't practice zazen. His point was just the opposite.
In zazen, we quiet our minds and stop the conscious separation of ourselves from the rest of potential. Being intimate with potential in this way is the ultimate expression of our existence, and it shouldn't be looked at as a means or a route to some greater good. It is in itself the greatest good.
It's not unlike saying that achieving immortality would be good for your health, or good for your complexion. If you're immortal, your health problems have pretty much been taken care of and there's no need to worry about sickness. If you're immortal, why worry about your complexion? You've already achieved the ultimate goal.
Same as with zazen. Yes, calmness and mindfulness will probably manifest themselves to some degree of another in your life, but so what? You only wanted calmness and mindfulness in order to become intimate with potential, not the other way around.
And saying the goal of zazen is to become intimate with potential is still limiting. As long as you hold a concept that "I" am over "here" and the "goal" is to be over "there," you're already so lost in conceptualization and ideation that intimacy with potential will elude you.
Just sit (shikantaza) with no goal in mind, and everything else will take care of itself. Zazen is good for nothing.