I agree that, despite mud-slinging from both sides on this debate, we have little empirical evidence to go by. However, I think that, in this absence of evidence, doing nothing could be just as risky and probably less noble than promoting accountability.
Let's look at the theory, as there is little evidence. For the people on the justice side, they must prove that prosecutions do more good than harm. Usually, the argument is made that prosecutions are the just thing to do, that they deter future violence and that the counter a culture of general indiscipline and impunity. Strong arguments, I would say. But prosecutions can destabilize as well as deter, and we don't know how much one Lubanga prosecution, for example, buys us in deterrence or destabilization.
Also, international prosecutions usually come in the single digits per annum - if we lock up 5% of the top-level killers (don't even mention the low level ones), will it really deter the other 95%? This reasoning, however, can be flipped around and applied to those worried of destabilization: will prosecuting 5% really bring down the regime?
For people on the destabilization side, however, I would say this: The impunity that was supposed to be the glue of the transition in the Congo has ended up being more like acid. The fact that Rwanda was not held responsible - in the court of donor politics or in an international court - for its crimes in the eastern Congo gave it a blank check. Same goes for Kabila's crackdown on the Bundu dia Kongo in Bas-Congo in 2007, or the rash of killings and rapes by Congolese army members in the Kivus. It is hard to argue that impunity has been very successful in stabilizing the country.
I would also emphasize that this is not an either-or situation. Vetting, for example, would allow Kabila to purge his army of abusive officers without having to try them in a court of law - they would be forcefully retired, as happened in many post-Soviet countries. A real Truth and Reconciliation Commission (not the farcical one we had during the transition) could educate people about happen and serve as a warning to officers not to re-offend. Above all, a serious crack-down on current abuses would send a strong signal, especially if commander are held responsible for the behavior of their soldiers.
It is fair and well to preach justice, but it is worthwhile noting the hypocrisy of western donors: After the US Civil War, President Johnson handed Lee and his men a collective amnesty. In Spain, to this day no one has been tried for crimes committed during the civil war (an amnesty law was passed in 1977); no Italians to my knowledge were tried after World War II for their war crimes; no American has been sentenced for crimes committed at My Lai in 1968 and only a few for Abu Ghraib. The biggest exception are the Nazis and their allies, of whom thousands were tried throughout Europe and executed.
There is no formula for how to do this. But, when in doubt, I would suggest being smart about it, tailoring any solution to the local situation, involving local civil society groups (two hundred of whom demanded a tribunal), and erring on the side of justice.