Also, this line of argument struck me as a sneaky piece of Dark Arts, though in all likelihood unintentional:

Countering the counterargument that morality is too imprecise to be treated by science, he [Sam Harris] makes an excellent comparison: "healthy" is not a precisely defined concept, but no one is crazy enough to utter that medicine cannot answer questions of health.

Actually, in the overwhelming majority of cases, "healthy" is a very precisely and uncontroversially defined concept. Nobody would claim that I became healthier if I started coughing blood, lost control of a limb, or developed chronic headaches.

However, observe one area where the concept of "health" is actually imprecise and controversial, namely mental health. And guess what: there are many smart and eminently sane people questioning whether, to what extent, and in what situations medicine can legitimately answer questions of health in this area. (I recommend this recent interview with Gary Greenberg as an excellent example.) Moreover, in this area, there are plenty of questions where both ideological and venal interests interfere with the discussion, and as a result, it's undeniable that at least some corruption of science has taken place, and that supposedly scientific documents like the DSM are laden with judgments that reflect these influences rather than any real scientific knowledge.

So, it seems to me that properly considered, this example actually undermines the case it was supposed to support.