Licensure requirements for medical and dental practitioners are another form of restriction of the free market, and hinder the ability of patients to seek services from willing providers.
Maggie Koerth-Baker shows how dental licensure requirements are harming patients in rural portions of America in her post, "Armed To the Teeth".
And of course, similar arguments apply to medical licensure. For more information, see this discussion from economics professor Shirley Svorny of Cal State University Northridge.