As much as we complain about the justice system, legitimate complaints, I think that its owed at least to the families of the victims to know WHY the defendant did what he did. He has the right to defend himself. And he should vigorously do so, if he believes he's not guilty. Military court unlike civilian courts have much more hefty sentences. There is no such thing as parole under the UCMJ. The best you can hope for is a reduced sentence if you commit a crime as an active duty soldier, sailor or airman.
Also this exercise of giving him a trial is to give transparency to the miitary court system, which is also the right thing to do.