I would say that anytime that it appears that we are selectively attempting to enforce laws, it harms our credibility. Whether it harms our interests is situation dependent. For example, let's say that waterboarding KSM was illegal, but that we got some actionable intelligence to prevent a significant attack upon us. I think you could argue either way whether this was in our best interests (good in terms of defending against a threat, bad in terms of public diplomacy).
Good point.

So where are you going with this line of questioning?
IMO, Chertoff's proposal would simply empower our rivals by giving them a legal instrument that we claim to support. The Russians did this exact thing with Georgia using the Kosovo precedent we set. The proposal by Chertoff was not necessary for US/NATO action in Afghanistan -- what has changed to make us need to use it now? And if it's not necessary (read: there are alternatives to justifying US action), why bother legitimizing the intentions/actions of our near-peer competitors? I also wonder what the consequences are at home in forming domestic concensus on policy when we make these kinds of institutions/laws based on moral arguments, only to selectively apply them.