4. If modern medicine is research proven and/or based on best practice standards, why is knowing about this important for people with DD? 

Unfortunately people with DD have not benefited as much as other populations from the scientific revolution in clinical treatment. There are many reasons for this, but what it means for treatment is that most medical and mental health professionals have little to no experience providing treatment to people with DD. Many will not be familiar with, or perhaps even be aware, that the treatment standards for people with DD are different from the general population. 

In addition the scarcity of experienced professionals can cause parents, guardians, and professionals to turn to unproven treatments. There are many of these "fad therapies" out there - many promising miraculous results. Unfortunately these are rarely effective, and often are harmful. Even those fad therapies that do not cause direct personal harm can cause indirect harm in the form of opportunity costs - meaning that they waste time, money, and other resources that could have been dedicated to proven treatments.