I feel like this is a passive attitude that we all tend to take because we have little idea of how MMIs are rated.
If someone actually knows the rating criteria, I'm sure there would be ways to prepare.
For instance, if cogency of thought can be determined by how thorough your initial response to the scenario is, it would behoove applicants who are normally reticent to push back against urges to merely be concise. In MMIs, perhaps it would be better to vocalize all the different possibilities, pros and cons, contingencies, of each scenario upfront, rather than assuming and dismissing some internally and then bluntly stating your position. The more prompting questions that the rater has to give you, the more indicative of your incompleteness of thought. Maybe.
For the few ethical scenarios there are, helpful frameworks exist for thinking about your answer. You seem more competent when you can organize your thoughts about some ethical dilemma into neat categories like, 1) what's good for the patient, 2) what allows the patient most autonomy, 3) what's most fair and just, 4) are there social/legal context considerations.
It also might be helpful (but lower yield) to read generally about medically-relevant topics like giving bad news, overcoming guilt, overcoming addiction, cultural competency, insurance, pharma, malpractice, evidence-based medicine, role of residents and students in care, discrimination, medical errors. For many of these, I felt it helped a little to read the health/medicine columns in the NYTimes.