Yes, because medical science isn't settled science and there is no "one right way" that is effective and well-tolerated every patient. The idea of denying coverage based on how cooperative a patient is with treatment reduces patients to a set of numbers, erases quality of life issues, and takes individual preference/comfort entirely out of the equation. If insurers can deny coverage based on medication refusal, we create a situation where one has to accept whatever side effects come with whatever the doctor has prescribed (and the insurer has approved). So your blood pressure might be normal, but you experience such dizziness/vertigo that you can't function, or you cholesterol might come down while you experience near-constant muscle pain/weakness, or your mental health might be more stable at the expense of large weight gain or sexual dysfunction. People shouldn't be forced to accept those kinds of trade-offs for the sake of insurers' profit motives.