In Act III of All My Sons, what is shown by the fact that Jim has always known of Keller's guilt, but kept silent?

This is a highly timely and important revelation, as Jim's
understanding of Keller's position and his own revelation about how he has been
similarly involved in acts of compromise underscore the theme of moral compromise due to
monetary greed. Keller is not alone in the world having done what he has done, as the
character of Jim Bayliss makes perfectly clear. Note what he says to Kate in order to
console her and to explain how he knows the truth about her
husband:

We
all come back, Kate. These private little revolutions always die. The compromise is
always made. In a peculiar way. Frank is right--every man does have a star. The star of
one's honesty. And you spend your life groping for it, but once it's out it never lights
again. I don't think he went very far. He probably just wanted to be alone to watch his
star go out.

Note how his own
struggles with precisely the same issue make this central theme of moral compromise for
money a universal issue. Compromise is accepted as a fact of life that cannot be
ignored, but Jim's speech above makes clear the devastating consequences for those who
do allow the light of their star of honesty to become
extinguished.