Harm can be difficult to prove and regardless of harm done, it can decrease trust in medical professionals. Spreading intentional misinformation should be considered a sign they are incapable of doing a job that so heavily relies on scientific research and practice.
After harm from the patient occurs it would cost the patient, doctor, and possibly government (if they get involved), and by then harm has already been done. It's better when advice comes with evidence that makes it sound and trusted.
