I wonder if the fact that DNA & RNA only occur with one chirality tells us something about how frequently this step occurs in the Drake equation. If the step from non biological organic molecules to RNA & DNA happens often, wouldn't we expect L-DNA and L-RNA to appear in nature?

It is pretty strong evidence that the step from going from other molecules to RNA/DNA only happened once. Our molecules definitely changed their environment, so it is probable that they out-competed their mirror molecules.