The Least Bad Option

Eric Posner sorts through the KSM trial spin:

The main criticisms of Holder’s approach are that KSM and others will take over proceedings and use them for propaganda purposes, that secrecy will be compromised, and that the approach signals insufficient seriousness about the terrorist threat.  The first two concerns are actually irrelevant.  The DOJ will decide on a case by case basis, and if those concerns in any particular case are serious, it will opt for military commissions.  The last concern is harder to evaluate, but it boils down to the claim that a blunderbuss system that results in outcomes that people distrust is better, on symbolic grounds, than a surgical system that produces the same pattern of convictions but with higher overall credibility.  Why would the more intelligent approach signal lack of seriousness about terrorism?