Mental Health Advocay in LA spends most of its time proving to SSA that their clients cannot work due to depression, schizophrenia, paranoid delusions etc. Many people who are emotionally disturbed become addicts and perhaps many people who are addicts become disabled, I am not an expert but there is sometimes a relationship. Some people drink to numb their emotional or mental condition, that is, self-medicate. This can lead to addiction. Many are without proper medication (not accurately diagnosed much less have the luxury of getting the right drug in them). But addiction is no longer a qualifier for SSI. Since many of these people find themselves in financial destitution they often do not have the means to get the "proof" necessary for SSA - meaning doctor reports etc because of homelessness or inability to act logically in their own interest. Mental Health Advocates fill in the gap.
I just read Doyle's post, so I won't go on. He is very clear.