But what usually happens is that people are required to be clean for a long period of time before they're allowed to access therapy at all. This model, to put it bluntly, sucks

Yeah, I personally think that's dumb. Someone with dual diagnosis (mental health/substance abuse issues) needs both counseling and specialist treatment. On the flip side I've known people who were in therapy for long periods of time who kept abusing drugs, and it really got in the way of their dealing with any other issues. Dual diagnosis also sucks.