Practically: Not so important. There are very few retiree cases.
Symbolically: Hugely important. Larrabee shows that the talismanic invocation of "good order and discipline," absent a rational underpinning, is insufficient to decide the limits of military jurisdiction--even if Congress is the institution invoking it. The judiciary's response: "Please!"
We will be sure to watch the inevitable appeal and keep readers updated on its status.