I respectfully disagree. First of all, would you ever try to justify an action as moral by saying, "The lawyer said it was ok"? I think that we all know that what's legally permissible and what's morally permissible can be very different. In almost all cases, they do correspond with each other, but they may not always, and they come from different sources. Law and policy is contingent--written by a lawyer or commander; morality is necessary--it has a less flawed, more enduring Author. Thus, for a soldier who has killed someone in Fallujah, the knowledge that the JAG in Baghdad or Arlington would say that the killing wasn't illegal isn't going to be a great comfort to his conscience. He already knows that, since he acted IAW ROE, he won't be court-martialed; but he may still wonder if what he did was morally wrong.
I dispute the claim that Soldiers who are familiar with the justification of killing in war would undermine discipline by being more likely to hesitate and put themselves and others in jeopardy. For one, a well trained Soldier will react reflexively to an imminent threat, without having to think, regardless of his knowledge about morality. Second, in those ambiguous situations where a Soldier must think and use his judgment, a well formed ability to reason morally will aid him in making the right decision, faster. Third, if a proposed action that is permissible under ROE turns out, in a particular sitution, to be immoral, we WANT to have Soldiers who are sufficiently morally aware to make the morally correct decision.
In the end, though, I just can't see a Soldier reflecting--days or years after killing in war--on his actions and saying, "I did what was morally right; I acted within the ROE." I HAVE heard statements like that, but they ring of rationalization and guilt, not justification and peace.
The ROE are the "cheat sheet" that provide the basics for conduct in combat, but I'm convinced that a deeper appreciation of the moral principles that are prior to--and foundational for--the ROE will better support Soldiers' long-term development as Soldiers and as people.